Porpoising/Hydrofoils

[fish45]

Subject: Hydrofoils for small boats with midrange horse power.
I have a 2012, 16 foot Smokercraft with side console controls and a 40hp Yamaha. My problem is with porpoising. With the motor trimmed all the way in (bow down) the boat travels quite well but when I start to trim the bow up so I have less hull in the water my speed starts to increase but the porpoising starts. The more I trim the engine out the worse the porpoising gets to the point where it gets a bit scary. I have been looking at the ?StingRay? Starfire Hydrofoil?. I have been looking at this one because I do not want to drill holes in the cavitation plate. So . . . my question is . . . have any of you guys used hydrofoils to control porpoising on a boat and motor of this approximate size or do you think a hydrofoils are a waste of time and I should just live with what I?ve got? I would really appreciate your opinions.

 

[smokeonthewater]

Re: Porpoising/Hydrofoils

two words

smart tabs

 

[saumon]

Re: Porpoising/Hydrofoils

What he said.

Hydrofoil, DoelFins or WhaleTail (which is three different names for basically the same thing), CAN, in some cases (particulary with an underpowered setup), help getting on plane faster but on others it do nothing and can even cause handling problems. It CANNOT help with a porpoising problem cause, when on plane, the anti-cavitation plate must be out of the water.

Porpoising is caused by the prop losing his grip, thus failing to keep the bow higher. I'd first start with playing with the engine height. How's your AV plate in relation to the bottom of the keel (both being measured level)? A good starting point is to have the AV plate 1.5 inch higher than the bottom of the keel...

 

[64osby]

Re: Porpoising/Hydrofoils

On my old boat, 16' FG with a 50hp Merc, it would porpoise at WOT, no matter how the motor was adjusted. Bought a whale tail, the plane speed was improved, but it still porpoised. Never tried smart tabs, but I hear they are great.

I solved my problem in a low tech way, added 50lbs of sand bags under the bow. Never had another problem. Not a great solution to the problem but it worked for me.

 

[rallyart]

Re: Porpoising/Hydrofoils

I'm a fan of foils but they do not stop porpoising. You need weight in the bow or something to change the shape of the hull to fix the problem. Smart Tabs are a reasonable choice for a small boat, or adding a bit of 'hook' near the transom. Try weight first. It's cheap.

 

[Texasmark]

Re: Porpoising/Hydrofoils

What about increasing your throttle setting? For a given speed/load a simplex hull, especially on light boats, will tolerate only so much tilt angle to the engine. Once above that angle you will get porpoising.

My boat will do it easily. Example. I will plane out with some throttle/trim setting and be on my way. If I get it I have two choices: Speed up or tuck it in. Simple fix.

Another example is I am running along at WOT or thereabouts hull running nice and trim, clicking off some serious mph and decide to cut her back to mid range rpms. Immediately it starts it and I merely tuck it in till it quits. No big deal.

Try it.

Mark

 

[fish45]

Re: Porpoising/Hydrofoils

Thanks for your help guys. Like I said in my original thread, the boat runs great with the engine is trimmed all the way in (no porpoising). What I was hoping for was to be able to trim the bow up so I would have less hull in the water to increase fuel economy. If hydrofoils will not reduce porpoising then I do not want one. The hole shot seems good as is. The AV plate is exactly level with the keel so that sure could be part of my problem. I will try the cheap approach of adding weight up front (my duck hunting buddy is a big guy). Guess I?ll just live with what I?ve got or check into getting Smart Tabs. From what I have been reading . . . they could be just the ticket. Thanks again for all your comments. Sure glad there are guys out there that are willing to help.

 

[mnypitboat]

Re: Porpoising/Hydrofoils

Joining late. I concur with smart tabs. My boat, albiet a larger boat, porpoised at full trim. I could eliminate it by triming back down but then it felt like it was plowing. I installed Smart Tabs and it completely eliminated the problem. I can trim up nicely now. Well worth the money.

 

[WIMUSKY]

Re: Porpoising/Hydrofoils

Sounds like you have a new boat? Have you talked to the dealer? May not of been set up properly. Sounds like the motor is too low for starters. What's the max HP rating? Usually that style of boat doesn't use tabs. At least I never see them. Could be a weight distribution issue. You could be tail heavy. Are you running a 4 stroke? Can you "move" any weight forward rather than add weight?

I have a 4 stroke 60 on my 16'. The 21 gal fuel tank is a little too far back(Crestliner has since fixed the issue). I am tail heavy. When I trim it out while on plane, the needle is at the bottom mark on the gauge. Any more trim and it will porpoise. Thankfully I can trim it out enough to get the bow up and most of the hull out of the water. When tucked all the way in the needle is below the lowest mark.

When my daughter(120#s) sits in front of the console and uses it as a backrest, the boat performs perfect.... I can then give it more trim...

 

[Outsider]

Re: Porpoising/Hydrofoils

Porpoising occurs when the lifting moment on the bow ceases to be =/> than the weight to be lifted, the bow falls, lifties take over again, repeat ad nauseam. With that said, some boats (most noticeably small boats) tend to be bow heavy and don't respond well to raising the bow. On a 16', you might as well just trim the engine as far as possible without porpoise and go on. Either one holds the bow down, so why go to the expense of tabs ...

 

[JimS123]

Re: Porpoising/Hydrofoils

Porpoising occurs when the lifting moment on the bow ceases to be =/> than the weight to be lifted, the bow falls, lifties take over again, repeat ad nauseam. With that said, some boats (most noticeably small boats) tend to be bow heavy and don't respond well to raising the bow. On a 16', you might as well just trim the engine as far as possible without porpoise and go on. Either one holds the bow down, so why go to the expense of tabs ...

Finally sensible advice. I always cringe when somebody says add weight to the boat. Smart idea - make it go slower. I usually try to take weight out so it performs better.

 

[UncleWillie]

Re: Porpoising/Hydrofoils

Porpoising occurs when the lifting moment on the bow ceases to be =/> than the weight to be lifted, the bow falls, lift takes over again, repeat ad nauseum. With that said, some boats (most noticeably small boats) tend to be bow heavy and don't respond well to raising the bow. On a 16', you might as well just trim the engine as far as possible without porpoise and go on. Either one holds the bow down, so why go to the expense of tabs ...

+2
Stop Trimming up so far!
Tabs add the ability to Tim DOWN even farther than the engine can give you.
Trim Tabs DOWN, and Engine UP, is like running the Heater and the Air Conditioning at the same time.

You will be able to trim the Engine Up more, because the Tabs are producing more down.
The only advantage will be a slightly longer effective hull length.
The more "Stuff" you put in the water the more drag it will produce.
There is no free lunch.

As you trim the engine Up the amount of hull in the water Decreases but the Depth that it is in the water Increases.
Once you pass the sweet spot, Porpoising starts and speed goes down.

If full up trim causes Porpoising, you have enough up.
If full down trim does NOT stop hull slamming in the chop, you need more Down trim, and Tabs are indicated.

 

[64osby]

Re: Porpoising/Hydrofoils

A 16' boat doing 33 mph is not exactly performance.

The 50 lbs added in the bow allowed the motor to go WOT and the boat to hit max speed with out bouncing all over the lake.

 

[CheckmateEnticer]

Re: Porpoising/Hydrofoils

I'm no expert but my boat seems to porpoise worse since removing my hydrofoil. I have a 16'6" with a 140. What I do to help is accelerate and then slow down while adjusting trim. It will porpoise while cruising mid 30's but I can accelerate to mid 40's to stop it then adjust trim as I slow down. For some reason I can cruise at 30's or even less after slowing down and adjusting trim. Make sense? Doesn't to me either but it works for me.

 

[UncleWillie]

Re: Porpoising/Hydrofoils

I'm no expert but my boat seems to porpoise worse since removing my hydrofoil. I have a 16'6" with a 140. What I do to help is accelerate and then slow down while adjusting trim. It will porpoise while cruising mid 30's but I can accelerate to mid 40's to stop it then adjust trim as I slow down. For some reason I can cruise at 30's or even less after slowing down and adjusting trim. Make sense? Doesn't to me either but it works for me.

Why speed up!?
Just Down Trim when the Porpoising starts.
You'll end up at the same speed with the same trim without the speed up, slow down, routine.

 

[JimMH]

Re: Porpoising/Hydrofoils

Smart Tabs

 

[ufm82]

Re: Porpoising/Hydrofoils

Porpoising on boats your size and horsepower is probably one of the most common complaints from owners. Often times boats this size are first-time boats and the owners do not know that characteristics inherent to these boats. That's not an insult to new owners- it's just not something most people have experience with.
The post Outsider made is actually the best of the bunch because it identifies the cause of the problem. The ability of the prop to lift the bow is insufficient to keep the bow up. It lifts it until the weight is simply too much for the prop to hold up and it falls. The weight is relieved and the cycle repeats. The only way to decrease/prevent this is to increase the lifting ability. Tabs can help. Foils, if the engine is installed properly, won't help because as noted, they are out of the water while the boat is on plane.
The single biggest issue with the boat is hp. A 40 hp engine and its associated prop simply cannot lift the bow of the boat. If you could turn a prop with better "bite" you'd see a difference. Rake, cup, pitch and other factors all enter into the equation. Unless you can hit that magic combination of power, bite and weight distribution it can be very frustrating. Most "fixes" are bandaids that don't address the problem. I share JimS123's viewpoint- I don't want to add weight to kill performance to prevent the porposing. You could simply just run the boat at 3/4 throttle and achieve the same effect. Instead, address the issue. Try moving weight around or even getting rid of some weight. Increase your ability to grip the water. Add horsepower. You get the idea.
I went through 7 props on my boat until I found the right one. Always trying to get the best grip and the best thrust in the same prop. I tried a standard Quicksilver prop, a Mirage, a Laser II, a Vengeance, a Tempest, a Vensura/Revolution 4 prop, and finally back to the Tempest. This prop allows me to trim up very high and get maximum speed but grips well enough to prevent porposing. The props with no cup ( Vengeance, Mirage, etc) all porpoised severely when trimmed up and the smaller diameter props (Laser and Rev 4) lacked power on my boat. The Tempest achieved a balance of all things I was looking for. And the prop is just a small part of the whole puzzle!
I find fewer complaints when people run the highest hp engine their boats are rated to handle. Just backs up my argument- power is the answer.

 

[Texasmark]

Re: Porpoising/Hydrofoils

Why speed up!?

For me the option is what I feel like doing at the moment. Sometimes one other time the other. Personal choice.

Mark

 

[Texasmark]

Re: Porpoising/Hydrofoils

In all this expert testimony, nobody including me addressed the possibility of a HOOK in the hull. So I'll address it.

Take a straight edge and starting at the transom run it as far forward as it will go...the farther the better.....till the bow V starts forming. If straight fine. If along the way there is a gap between the hull and the edge you have a hook and you probably will not solve your problem with anything but a new hull.

Mark

 

[CheckmateEnticer]

Re: Porpoising/Hydrofoils

Why speed up!?
Just Down Trim when the Porpoising starts.
You'll end up at the same speed with the same trim without the speed up, slow down, routine.

That is with the trim all the way down. I don't trim up at all until the poroising has been tamed. I don't understand it either but that is the best way for me to find the sweet spot that results in the lowest cruise speed without porpoising.