outboard sucking air on plaining

[sameh]

Hi
I have a 3.8 meter zebec SIB with mercury 15 hp short shaft, and i a few questions that may be related one to the other:

1- i have a 4 part wood floor. where should the side rails be fixed? starting from the rear touching the transom or as i was told from half the second to the third( the one touching the transom being the first)?

2-used to have 6 hp outboard but no plaining and no fast speed. now with the 15 hp with two persons onboard i reach 30km/h but sometimes after plaining starts the outboard seems to such air and make false high rpm with no much response of the boat. It may be happening if the boat if not inflated to the maximum or in choppy sea but i just cannot connect it to certain reason. could it be possible that a SIB needs a long shaft?!! boat not fully inflated? side rails in the wrong place? ftubes get farther of each other during time and floor not tight enough?

3-what is the best way to get on plain?the one in front should be at the bow until boat plains then comes back to the bench? should i change something with gas at some stage? is it natural that inflatables loose plaining in choppy sea very often?

alot of questions alot of thoughts..

would be happy to provide any information needed.

would appreciate your help

 

[Sea Rider]

Strange a 3.80 Mtr Sib having only 4 wood pannel boards when ideal is 5. Bow panel must be completely free to forn an angle when keel is inflated. Are side joiuners 2 piece per side or one long per side. That 3.80 should have a Owner's Manual, check wooden panel installation procedure. If side joiners are 2 piece per side with different lenghts, must be installed at an X pattern, that's short/long port side, long/short stardboard side right in middle 3 wooden panels.

To get the best of any inflatable Sib, must be air topped to at least 3.0 PSI on all air chambers, keel included, the only way to achieve correct hull rigidity. Get a pressure gauge and a good hand air pump. Can go from there optimizing Sib's water performance..

Happy Boating

 

[sameh]

Strange a 3.80 Mtr Sib having only 4 wood pannel boards when ideal is 5. Bow panel must be completely free to forn an angle when keel is inflated. Are side joiuners 2 piece per side or one long per side. That 3.80 should have a Owner's Manual, check wooden panel installation procedure. If side joiners are 2 piece per side with different lenghts, must be installed at an X pattern, that's short/long port side, long/short stardboard side right in middle 3 wooden panels.

To get the best of any inflatable Sib, must be air topped to at least 3.0 PSI on all air chambers, keel included, the only way to achieve correct hull rigidity. Get a pressure gauge and a good hand air pump. Can go from there optimizing Sib's water performance..

Happy Boating

Hi luis
I apologize, checked again and it is 5 wood parts and not 4. you were right.
it is a one piece side joiner. attaching some photoes for that.
So, do you exclude a short shaft being the cause for sucking air or that could still be an option?
​anyway, I will follow your recommendation and see how it goes.
trying to upload pictures but failing so far!

 

[Sea Rider]

Sameh,

That Sib must have a short transom height that should match perfectly a short shaft OB. Anyway, measure the height distance from upper middle transom to lower transom (keel) in straight line. Should be around 38-40 cm in height.

Moons back wrote this tech essay for UK boaters across the pond, read it fully to understand some basic principles that will make you boating life more enjoyable. If any point is not clear to your boating understanding just ask.

http://www.rib.net/forum/f50/abc-sib-rib-installation-guidelines-58373.html

Happy Boating

 

[sameh]

Sameh,

That Sib must have a short transom height that should match perfectly a short shaft OB. Anyway, measure the height distance from upper middle transom to lower transom (keel) in straight line. Should be around 38-40 cm in height.

Moons back wrote this tech essay for UK boaters across the pond, read it fully to understand some basic principles that will make you boating life more enjoyable. If any point is not clear to your boating understanding just ask.

http://www.rib.net/forum/f50/abc-sib-rib-installation-guidelines-58373.html

Happy Boating

many thanks sea rider
for some reason i wasnot notified about your reply. I will do the measure and update you with my findings.
thanks
sameh

 

[sameh]

No Title

o,k,, couldnot sleep without making measurements: so, indeed it is a 39 cm transom. the keel reaches the outboard above the cavitation plate-as seen in the picture at the wood level. so, it seems that the problem does not exist there i assume.

 

[Sea Rider]

Sameh, was wondering what does sucking air means, probably having prop aeration due to OB sitting way high on transom or badly trimmed. Tomarrow will modify your posted pic and post it back with some parameters to have into consideration.

Happy Boating

 

[Sea Rider]

Sameh, your posted second pic is very small, just 120 x 90 stamp size pic, impossible to work with. Shoot same one at least in 2 megapixels/JPG format. BTW, how new or old is that 15 Merc OB, is it a 2 or 4 strokes OB ?

Happy Boating

 

[sameh]

No Title

Hi Sea Rider
attaching better pics now. are they o.k.? for some reason i cant upload JPEG unless converting to PNG!
I was meaning that the engine creates a high sudden sounds (as if operating at high rpm on neutral) without boat plaining or increasing speed..that disappears if i slow down untill the propeller grips in the water!

the outboard in a japanese version of mercury 15 hp. it is a 2015 new 2 stroke short shaft outboard. comes with gear on the side and not on the tiller, it is about 12 kg heavier than american 15 hp but a bit cheaper and probably stronger..i think it is parallel to tohatsu 18 hp! it's name is 15M JP, engine size 294cc (in comparison to 234cc in the american type as i remember)

 

[Sea Rider]

You have a rebadged Tohatsu 15 Hp OB as per seeing lower leg shape, excellent OB. Have you read the link sent ? need to concentrate on the second post titled Boat Balance, read if fully, duplicate that test on yours. You can have 3 issues : Spun prop hub ? OB transom height does not match well, badly trimmed.

Trim OB as indicated, go for a wot spin with mate sitting up front, pull head out transom, visually check where water flow is passing at speed on calm flat no wind water cond, if prop aerates badly, reduce grip till props bites water again while maintaining full horizontal plane. If it's not a spun hub, 2 possible aeration posibilities :



Best performance is achieved when water flow at speed skims right under green line, if passes through yellow one will have some form of prop aeration, at red line will give an awful excessive aeration. Need to determine firstly before moving ahead at which leg height is water flow passing at speed. Do you boating homework and report back your findings LOL!!

Happy Boating

 

[sameh]

No Title

thanks SeaRider
I have read it all but need to go over due to some unfamiliar hard phrases. by the way, what is the meaning of what you wrote: "Spun prop hub"?

Ofcouse I will do the test when boat fully inflated in water and try to get video .

I have two more question regarding side rails (pictures attached)
First, should the side rail at the transom side start somewhere near the middle of the black patch on the tube? As someone explained to me.

Second, the side rail has upper side and lower side. One of the sides is broader. At the moment, I have the rails where the wider side is from below, where the narrower side is from above the plain of the floor, hence the bigger area is from below. Seeing some videos, it may be wrong, where the narrower side should be from below. I would appreciate if you can give your opinion about that, including the logic behind that...

 

[Sea Rider]

Sameh, spun hub means, that vulcanization of the inner hub has become bad, prop not gripping right. Like a car clutch when going bad. Is side joiners seen on first pic that short. Should have another patch beginning bow alum floorboard. Side joiners must match middle of both transom/bow glued patches. Should have 2 shorter, 2 longer side joiners, do you have them ? The wider part of both rails should be assembled facing lower floor fabric.

Asume if placed backwards the round part (side of them) will not rest properly on side tubes once tubes are inflated, even could pop out floor when sib is on plane on choppy waters. Let's resolve the basic and once you have dialed the sweet transom/OB height match, we'll check about prop.

Happy Boating

 

[sameh]

hi
problem solved! and it came from a direction i did not think of at the beginning:

the boat trim was on second lowest point! I bought it new outboard in the box and assumed it will be at the lowest point and did not check that! all the time i was boating when outboard is not on the lowest point while thinking it is!
after reading discussion here, i had another look and found out about it. I brought the outboard to it's lowest trim point and during 2 boating trips (about 25 miles together) i did not have a problem not even for a second, and the boat went to plaining relatively fast (although i had to give it full throttle to plain at beginning in some times where i had choppy sea on both occasions, a 25 hp would be even better but heavier and non realistic to put it on and off the boat each time with its heavier weight, and boat reached 32 km/h on choppy sea of 5 feet!
so, problem solved. thank you very much for your support that helped me think out of the box.

 

[Sea Rider]

Sameh,

If trim is set on the low side bow will tend to dig downwards against water level. You should trim for combo to ride parallel to water level when on plane, usually second/third trim hole out transom. Have you checked at which tail height is water flow passing at speed, need to pull head out transom when on plane to check that, provided that your are at the correct trim setting as suggested previously.

Happy Boating

 

[sameh]

Sameh,

If trim is set on the low side bow will tend to dig downwards against water level. You should trim for combo to ride parallel to water level when on plane, usually second/third trim hole out transom. Have you checked at which tail height is water flow passing at speed, need to pull head out transom when on plane to check that, provided that your are at the correct trim setting as suggested previously.

Happy Boating

hi
yes, I have checked. on plain water level is on the green line!

you are right that the bow is low but i don't have a feeling that it digs rather being parallel to water. unfortunately second hole caused me problems and third one would probably cause more problems, so I am satisfied with the current situation right now..

 

[gwozhog]

Great advice Searider and excellent pic. I was having the same problem on my Achilles when I had 600 pounds worth of passengers with my 15 hp. I had been using a small 3/4" board on the transom to raise the outboard and turns out it was causing air syndrome. Ran like a champ when under 600 pounds but struggled when over. I removed the board and it performed much better having the motor sit deeper in the water.

 

[Sea Rider]

Good to read that have worked OK for you guys. Checking what's going on out transom is a must do to dial best OB height and trim setting. Not all boats transoms angles are same, will vary depending on the manufacturer specs.

Happy Boating