Hydrofoil for Inflatable Seamax 430

[SEAMAX 430]

I have always had a very significant problem with excessive prop ventilation on my Seamax 430 inflatable. I am running a 20HP Honda and it seems that it is impossible to get the boat into any kind of nose high attitude even at start. When attempting to trim for a nose high attitude the prop immediately ventilates, leaves the water and the boat comes to a stop. In order to operate the boat, it must be trimmed pretty much all the way in continually. I am new to inflatables and am wondering if anyone else is familiar with this situation. Might a hydrofoil like a Doel-fin or similar be of help. Any help gratefully appreciated. Thanks!

 

[JimS123]

I am an advocate of the DoelFin, having used them successfully on every boat owned since 1984. That said, I never owned an inflatable. My gut feeling is that it would be of no help, possibly even make the problem worse.

Post a picture of the motor height vs. the keel.

 

[SEAMAX 430]

I am an advocate of the DoelFin, having used them successfully on every boat owned since 1984. That said, I never owned an inflatable. My gut feeling is that it would be of no help, possibly even make the problem worse.

Post a picture of the motor height vs. the keel.

Thanks, that was kind of my thoughts as well that it may do opposite to what it is meant to do. I don't have any pix handy regarding the motor height but the cavitation plate is about 1-1/2 to 2 inches below the bottom of the keel. Will have to measure things up again. Common sense tells me that the motor may be a bit high because of the way it is responding. I have used the Doel Fins on some of my glass boats with some improvement.

 

[Sea Rider]

If the prop is aerating a doel fin won't cure the issue and will dump money into the deep blue. Is the motor currently sitting flat on transom ?

Correct starter procedure :

-Inflate Sib's tubes and lower keel to at least 3.0 PSI, if counting with a pressure gauge much better as finger touch monitoring is way off from where it should be if wanting a firm and solid Sib riding on water at speed.​

-Trim motor to 90 º usually second/third orifice out transom, brand sticker should be parallel to water level.​

-Sit a boater mate up front for better load distribution, go for a wot run on flat calm water condition... ​

pull your head side of motor and check at which lower leg's plates is the water flow passing once on plane. Can sit on deck close to transom if with fear doing same while sitting on aft tube portion. Take a pic if possible...

-Excessive prop aeration is due to an exposed AV plate as in pic, confirm the water flow is passing slight under or slight above the AV plate to go from there. Know that Sib brand and size well, a friend of mine had the same issue running a Tohatsu 30 HP motor on it.

Happy Boating​

 

[SEAMAX 430]

If the prop is aerating a doel fin won't cure the issue and will dump money into the deep blue. Is the motor currently sitting flat on transom ?

Correct starter procedure :

-Inflate Sib's tubes and lower keel to at least 3.0 PSI, if counting with a pressure gauge much better as finger touch monitoring is way off from where it should be if wanting a firm and solid Sib riding on water at speed.​

View attachment 339042

-Trim motor to 90 º usually second/third orifice out transom, brand sticker should be parallel to water level.​

View attachment 339043

-Sit a boater mate up front for better load distribution, go for a wot run on flat calm water condition... ​

View attachment 339044

pull your head side of motor and check at which lower leg's plates is the water flow passing once on plane. Can sit on deck close to transom if with fear doing same while sitting on aft tube portion. Take a pic if possible...

View attachment 339046

-Excessive prop aeration is due to an exposed AV plate as in pic, confirm the water flow is passing slight under or slight above the AV plate to go from there. Know that Sib brand and size well, a friend of mine had the same issue running a Tohatsu 30 HP motor on it.

Happy Boating​

Thanks for the info Sea Rider. I will start from square one when I set this boat up for the season, probably next week. From any information that you have passed on to me, I've tried it. With the motor set at 90 degrees to the water, the nose plows and the motor ventilates and loses all thrust before being able to plane. The motor has to be trimmed in in order to plane and being that it is trimmed in, it planes relatively nose heavy. It seems to be responding oppositely to what I would expect it to do. My hunch is that the motor may have to be lowered in order to get the bow to rise enough to break the suction. That's my thoughts anyway. Once I'm up and running I'll have a good look at where the cavitation plate is running and let you know how I make out. Thanks again .. much appreciated!

 

[Sea Rider]

You're welcome,

The AV plate set at any height in a dry installation with respect to the keel is no indication that when the combo runs on plane/speed will perform as expected as no one has taken into the least consideration that a Sib will float more or less once on water.
Extremely bad water performance will depend entirely on the tube's diameter and the transom height each Sib manufacturer wants to play with and some of them don't know what they're building LOL!! These guys should test well their inflatables in all water cond before being released to the public.


Is it possible to measure the gap distance in between both upper and lower plates to calculate how much will need to lower the motor on transom.



This is the same Seamax 430 you have powered with a short shaft 2 stroke Tohatsu 30 motor running on plain flat no wind water cond, would you say that runs spot on as expected to run ?

Happy Boating​

 

[SEAMAX 430]

You're welcome,

The AV plate set at any height in a dry installation with respect to the keel is no indication that when the combo runs on plane/speed will perform as expected as no one has taken into the least consideration that a Sib will float more or less once on water.
Extremely bad water performance will depend entirely on the tube's diameter and the transom height each Sib manufacturer wants to play with and some of them don't know what they're building LOL!! These guys should test well their inflatables in all water cond before being released to the public.

View attachment 339190
Is it possible to measure the gap distance in between both upper and lower plates to calculate how much will need to lower the motor on transom.

View attachment 339191

This is the same Seamax 430 you have powered with a short shaft 2 stroke Tohatsu 30 motor running on plain flat no wind water cond, would you say that runs spot on as expected to run ?

Happy Boating​

Good Day again. Totally agree regarding manufacturers. I have looked at it over and over today and visually I do not see any issue. The Seamax 430 picture you have enclosed IS spot on as I would expect it to run. Mine, configured as your picture runs flat and keel heavy. I have enclosed a couple of pictures that may help as far as the upper and lower plates. Another thought is that I will play with the inflation in the keel once I am on the water. As Always .. Thank You Sea Rider!

 

[Sea Rider]

Good morning S430,

Visually speaking won't see any issue, the 430 Sibs have an oustanding eye catching cool looking. The issue is that the tubes are darn wide in diameter and the transom too high for those tubes, that means that the Sib will float higher on water and with a taller transom will rise the lower plates accordingly for the prop to work with a much higher water level and achieve its top thrust. Those 430 are ideal Sibs for rescue purposes, not so much as speed machines as one would imagine.

Thought that 20 HP would have at the most a 3.0 inch gap between both plates and has 4.0 same as the Tohatsu 30 HP shown, combo which runs nice balanced on flat water at plane speed say 1/2 throttle but if you throttle to max the prop will begin to aerate, worse on non flat say choppy windy sea water conditions.

Our 430 when flipped did not formed a good rear tent like with keel inflated to max 3.5 PSI of pressure, there was a kind of factory flaw on the fabric adjacent to the left transom area that didn't stretched well. Flip yours and check, hope the rear floor fabric stretches fully ?

In other brand Sibs the keel extends near touching the transom and will form a better shapped V deep under the rear hull which will flatten much better the hardened water flow exiting the lower transom.

What do you mean by keel heavy ? with how many passengers do you usually go boating with ? if solo boating do you sit on any of the rear tubes or on the provided rear bench ?

Can you post a pic as first one but showing the full left side of the lower plates configuration...

Happy Boating​

 

[SEAMAX 430]

Good morning S430,

Visually speaking won't see any issue, the 430 Sibs have an oustanding eye catching cool looking. The issue is that the tubes are darn wide in diameter and the transom too high for those tubes, that means that the Sib will float higher on water and with a taller transom will rise the lower plates accordingly for the prop to work with a much higher water level and achieve its top thrust. Those 430 are ideal Sibs for rescue purposes, not so much as speed machines as one would imagine.

Thought that 20 HP would have at the most a 3.0 inch gap between both plates and has 4.0 same as the Tohatsu 30 HP shown, combo which runs nice balanced on flat water at plane speed say 1/2 throttle but if you throttle to max the prop will begin to aerate, worse on non flat say choppy windy sea water conditions.

Our 430 when flipped did not formed a good rear tent like with keel inflated to max 3.5 PSI of pressure, there was a kind of factory flaw on the fabric adjacent to the left transom area that didn't stretched well. Flip yours and check, hope the rear floor fabric stretches fully ?

In other brand Sibs the keel extends near touching the transom and will form a better shapped V deep under the rear hull which will flatten much better the hardened water flow exiting the lower transom.

What do you mean by keel heavy ? with how many passengers do you usually go boating with ? if solo boating do you sit on any of the rear tubes or on the provided rear bench ?

Can you post a pic as first one but showing the full left side of the lower plates configuration...

Happy Boating​

Good Day Sea Rider, I would normally be on the water with myself and my wife who is fairly "light weight". She will normally occupy the rear bench and I use the right tube. By "keel heavy" I would mean that it has a tendency to feel that the boat is reacting as if it has a lot of water in the hull (it does not). It very much seems that the 430 that you are obviously familiar with is exhibiting many of the same operating tendencies as mine causing me to believe that this may indeed be an inherent design flaw. I see exactly what you are describing with the deepest part of the keel being very much forward from the transom. The material itself seems to have a very loose, sloppy fit across the entire area between the tubes. I am going to leave all "as-is" for now and play with pressures and loading believing that it may be more of a problem with the design and not something that I am doing wrong. Another out of the box idea I may try is a doel-fin or other small hydrofoil mounted upside down on the outboard. Yeh, I can almost feel you laughing here. It is after all one of my boats that does not get a lot of use so a little playing with ideas is not going to hurt and besides, the boat is very much usable just as it is. Thanks again (and again..) Sea Rider for the information, it is very much appreciated.

Happy Boating to You as well!

 

[Sea Rider]

Some issues :
Which air pump is being used to inflate the tubes and keel ? if manual, it's a hand or foot pump, if electric can you select the amount of inflation pressure ?

Regarding the unven stretched fabric, it's a factory flaw, these guys have not learned so far to glue well what needs to be glued spot on. Imagine how many 430 are being factory delivered with such fabric flaw.

Why don't you install the provided wooden seats so to sit yourself on the rear seat for a much comfy tiller driving and the admiral on the front seat right at middle, should cancell what you describe as keel heavy. Out of curiosity what do you expect to cancel or better installing a doel fin.

Happy Boating
​

 

[SEAMAX 430]

Some issues :
Which air pump is being used to inflate the tubes and keel ? if manual, it's a hand or foot pump, if electric can you select the amount of inflation pressure ?

Regarding the unven stretched fabric, it's a factory flaw, these guys have not learned so far to glue well what needs to be glued spot on. Imagine how many 430 are being factory delivered with such fabric flaw.

Why don't you install the provided wooden seats so to sit yourself on the rear seat for a much comfy tiller driving and the admiral on the front seat right at middle, should cancell what you describe as keel heavy. Out of curiosity what do you expect to cancel or better installing a doel fin.

Happy Boating
​

I am usually using an adjustable electric pump and set at 3.5 or 4 lb depending if I am moving from warm weather to cool or visa-versa. I only use the manual for a quick top up if the weather turns cold and the pressure is obviously low. Seats are always in it but always felt more comfortable on the tube but am going to try as you say putting the admiral up front and myself on the rear bench. Not real sure if I am serious or not about an upside down doel fin but maybe it may put a bit of down pressure on the rear of boat to help stop the ventilation. I am for sure going to try the weight and balance stuff and air pressures before I try anything "outside the box". Take Care

 

[JimS123]

Not real sure if I am serious or not about an upside down doel fin but maybe it may put a bit of down pressure on the rear of boat to help stop the ventilation. I am for sure going to try the weight and balance stuff and air pressures before I try anything "outside the box". Take Care

The purpose of any fin is to raise the aft end. That's why, considering your "problem", I recommended against it.

A fin is a fin. I'm not sure that the aerodynamic design will do the opposite if you run it upside down.

Your choice. It's a real cheep option, so if it doesn't work you haven't wasted much money. You can always save it for when you buy your next boat.

 

[SEAMAX 430]

The purpose of any fin is to raise the aft end. That's why, considering your "problem", I recommended against it.

A fin is a fin. I'm not sure that the aerodynamic design will do the opposite if you run it upside down.

Your choice. It's a real cheep option, so if it doesn't work you haven't wasted much money. You can always save it for when you buy your next boat.

Thanks JimS123. I have never looked closely at the actual profile of the fins but if it performs similarly to an aircraft wing which I am very familiar with, it may have an effect that may or may not prove useful. If nothing else it may well be worth the shits and giggles. I usually have 5 or 6 boats going at any one time and probably have one kicking around somewhere anyway. Will definitely post any results if I do try something this wild. My gut feeling is that I may have to live with this quirk and the boat is very usable as is. One other thought just in passing, this will be the first season that I will be using this boat in fresh water as opposed to the salinity of the Florida Keys. In theory the boat should not have quite as much buoyancy as in the salt and may react differently. Time will tell all. Thanks again for your input .. very much appreciated!

 

[Sea Rider]

Good to count with an electric air and foot pump, large Sibs must ride fully inflated to their working pressures to get the max out of them.

Don't buy any doel fin or any other fin type, save the money for some cold ones. Useless installing one on top AV plate when already having prop aeration issues which a fin won't cancel, a fin must ride parallel to the water level and fully submerged to work as was designed to work unless wanting to use it a a step latter to climb on board from behind the motor....

The issue with your current combo is that has an awful motor/transom match, the pic seen on post 6 corresponds to the maiden voyage of the 430 and 30HP motor out of their respective boxes. This is where the water flow was passing at plane right over the exposed AV plate where all the excessive prop aeration was being produced...​

The only possible way to make that 430 powered with a Honda 20 HP to perform as expected was to cut the transom down at least 1 full inch for the prop to go deeper, have a taller water bed, bite much better and achieve a much better prop thrust than when at the maiden voyage...
Cutting the Transom down....​

Check the new lower leg height with water flow passing right under the small upper plate. No need to center the motor on transom, just drop it down and secure both clamps...
​

The owner is so content with the water performance of his reloaded Seamax 430 that now uses it a lost more than his yacht. Check in your area if there's someone sufficiently skilled to do that kind of work, will be a day night boating difference guaranteed...

Happy Boating​

 

[JimS123]

Don't buy any doel fin or any other fin type, save the money for some cold ones. Useless installing one on top AV plate when already having prop aeration issues which a fin won't cancel, a fin must ride parallel to the water level and fully submerged to work as was designed to work unless wanting to use it a a step latter to climb on board from behind the motor....

​

It has already been stated unequivocally that a Doelfin (sp) will not solve the OP's problem. It's also clear that rubber boats perform differently vs. solid hulled boats.

"Fins" are often maligned because people don't understand their function and optimization techniques. Yes, they need to run parallel to the water, but running it submerged is why they don't work right.

 

[Sea Rider]

"Fins" are often maligned because people don't understand their function and optimization techniques. Yes, they need to run parallel to the water, but running it submerged is why they don't work right.

Fins are garbage collectors and manufacturers making good money with them, useless on portable motors with tall gap plates. What you need to do is : trim motor to 90º, distribute deck load evenly, have a spot on transom/motor match, install a maximized prop for whatever floats to run towards it's max wot rpm range dialed with a tach. At full throttle from a dead stop the combo will jump on plane as if jumping from a trampoline guaranteed. A Matter to dial those 4 mentioned points for spot on boating perfection.

Scenario : if a fin is bolted to the upper AV plate and achieves excessive prop aeration as explained and documented here with pics, need to lower the leg to correct that flaw, right, once done, question : will the same fin ride submerged or not ? A fin install in a short gap plate motor as long works parallel to the water level won't care if rides submerged or exposed...

Happy Boating​

 

[SEAMAX 430]

Good to count with an electric air and foot pump, large Sibs must ride fully inflated to their working pressures to get the max out of them.

Don't buy any doel fin or any other fin type, save the money for some cold ones. Useless installing one on top AV plate when already having prop aeration issues which a fin won't cancel, a fin must ride parallel to the water level and fully submerged to work as was designed to work unless wanting to use it a a step latter to climb on board from behind the motor....

The issue with your current combo is that has an awful motor/transom match, the pic seen on post 6 corresponds to the maiden voyage of the 430 and 30HP motor out of their respective boxes. This is where the water flow was passing at plane right over the exposed AV plate where all the excessive prop aeration was being produced...​

View attachment 339372

The only possible way to make that 430 powered with a Honda 20 HP to perform as expected was to cut the transom down at least 1 full inch for the prop to go deeper, have a taller water bed, bite much better and achieve a much better prop thrust than when at the maiden voyage...
Cutting the Transom down....​

View attachment 339373

View attachment 339374

View attachment 339375

Check the new lower leg height with water flow passing right under the small upper plate. No need to center the motor on transom, just drop it down and secure both clamps...
​

View attachment 339376

View attachment 339378

The owner is so content with the water performance of his reloaded Seamax 430 that now uses it a lost more than his yacht. Check in your area if there's someone sufficiently skilled to do that kind of work, will be a day night boating difference guaranteed...

Happy Boating​

Yes Sir, I do believe that we are exactly in agreement. This is exactly what I figured and would expect to improve my problem. It's hard to believe that a relatively new boat would be that poorly designed that it would require a drastic modification as this to perform as it should. You are right, the idea of an upside down or even right side up doel fin went out the door last night after looking at my daughters Honda 35 equipped with the fins. Not at all a practical application and impossible upside down and useless in the end. I am capable of doing this modification myself but may wait for a full transom replacement as I am noticing a couple of spots that are starting to get soft. I am confident and much more at ease that I am on the right path now. Thanks so much for all your time, pictures and explanations .. It is very much appreciated!

 

[SEAMAX 430]

It has already been stated unequivocally that a Doelfin (sp) will not solve the OP's problem. It's also clear that rubber boats perform differently vs. solid hulled boats.

"Fins" are often maligned because people don't understand their function and optimization techniques. Yes, they need to run parallel to the water, but running it submerged is why they don't work right.

Thanks JimS123, I am going to save the fins for one of my "hard" boats. I have a Sea Ray Laguna 210 walkaround with an inboard outboard that I feel may benefit from a set. Normally I would install a set of Bennett trim tabs on something like this but did not want to mess up a contoured transom. It performs reasonably well as is but a little help from a fin to get it out of the water just a bit quicker would not hurt it. Thanks for your time and explanations .. very much appreciated

 

[JimS123]

Thanks JimS123, I am going to save the fins for one of my "hard" boats. I have a Sea Ray Laguna 210 walkaround with an inboard outboard that I feel may benefit from a set. Normally I would install a set of Bennett trim tabs on something like this but did not want to mess up a contoured transom. It performs reasonably well as is but a little help from a fin to get it out of the water just a bit quicker would not hurt it. Thanks for your time and explanations .. very much appreciated

Bennett's are the cat's meow for "hard" boats with I/Os. "Fins" on an I/O are hit or miss. Certainly they will provide lift and faster planing, but it may also result in a slight loss of top speed because there is no adjustment available. For that reason, people pooh pooh them. Speed is everything, ya know! They will always run below the surface on an I/O.

A fin on an OB can be part of the optimization process, but the motor has to be raised to minimize drag and put the fin skimming right on top of the water. If you don't take the effort (GPS, induction tach, record keeping, etc)you won't like it either.

With over 60 boat years with fins on 6 boats I am not an expert, but I know a coupla things....LOL. What I don't know is what's what on a rubber boat. But I'm learning. So far, to me it seems that they are a whole different ballgame.

 

[Sea Rider]

Many moons back wrote an essay about tall plates gap motor installations for anything that floats which was shot right after posting with a lot of associated expletives by the big shots that likes boating with short plates gap motors found in large HP ones but keep throwing both motor types in same sack and positively assuming that both are installed the same way, which it's a tech fallacy. I'd like you to have a look, once you read it fully will have the answers why isn't the combo performing right as was expected. If you call the Seamax Company and tell them that their boat isn't performing right due to a transom issue will guarantee that will come up with an excuse as blaming the motor for such awful water performance. Will say that's the same transom being installed on all the 430 Sibs....

Happy Boating​