New custom landing craft performance problems.

Hello all, Awsome place you have here!

I need some help from the experts.

I laid out the specifications for my uncle's 34' twin outboard landing craft. The boat has some performance issues I need to fix.

I had the bright idea (seemed bright at the time) to have the motors mounted on 5' centers to help with slow speed maneuvering and for a shallower draft while beaching the boat. Totally within the recommendations in Yamaha's installation manual, all it says is the minimum is__ inches.

The problem occurs at roughly 2,700 rpm the propellers start pulling in air and loosing bite. The only way to get the boat on step is to power through it, 5,000+ rpm. Once the boat reaches 20 or so the props bite rpm's drop by a 1,000 or so and off you go. The engines reach their maximum rpm and 38 mph.

The motors have to be trimmed all the way down but can be bumped up a little when the boat is well on step. If they are trimmed up and you slow too much the props slip and you fall off plane.

What can be done besides moving the motors closer together and putting on long shaft lower units to get them deeper in the water?

You can't tell from the pictures but the motor height is spot on with the cav plate. The boat does have a delta pad to help the boat sit up rite if it gets stuck on the beach. The chine at the transom is only 3-4" inches under water when the boat is at rest. There is nothing on the hull in front of the motors to disturb the water

We pulled the one zinc off to see if it was contributing to the problem, no change. Super cupped props helped but no other attempt has been made. The boat only has 50 hours on it.

Thanks in advance for helping a new poster. Mods feel free to move this if it's more appropriate somewhere else.

TK

 

Is the hull lifting so much at 2700 rpm that it sucks air in from below those shallow chines for some reason? If not, then where is the air coming from ... the surface above the props?

If the props are still in solid water I don't understand how they can be ventilating. Maybe they are simply not deep enough, in which case hydraulic jackplates might be the answer. They will let the skipper lower the engines before they reach 2700 rpm. The shallow water fishing guys along the Gulf Coast use jackplates on their tunnel hulls to actively control the propeller depth with apparent success, maybe they would work here too.

 

AWESOME BOAT!

What kind of cargo will you carry? Deck seems kinda small? But then, I think in terms of building materials..

Sorry I can't be of much help with the OB's.

 

Not a power-boater but I have seen some large delta shaped wings which are attached to the flat plate just above the prop. This I see quite often and it apparently cures the "sucking air" problem, unless the air is coming from the flat bow skid - feeding the propeller with bubbles...

 

I'm not a power expert, but I think the 'jackplates' could be the answer.

Moving the engines closer together would not be the answer.

 

My experience has been to lower the motors at most 50 mm. check this out they seem to work well but your motors are very big. i found at http://www.c-brats.com/portal.php?sid=722afaf60f624d79ac91e0e30563a18a

 

That is the thing I was talking about... Thanks Manie... I have also seen 3 times as wide in a delta...

 

Thanks for the replies!

It's hard to figure where the air is coming from. Here's my guess. I suspect since the engines are not very deep that the props pull water from the easiest place first. While at sub planing speeds could water replacing water that is displaced by the hull curl up from under and from the sides filling the void left by the displaced water and mixing with air then pulled in by the props? It really boils back there and you can't see anything but frothy water.
That was kind of wordy, I'll try again if not for yours but for my sake. Say the boat is half on step and the water is breaking away from the transom leaving a hole that is filled by turbulent aerated water that is pulled down to the bottom then it's at the cav plate level and gets sucked in by the props?

Whew! Clear as mud!?!

I thought of jack plates but the owner is not very boaty and I need to keep things simple but they are an option.

The boat has a 14* V so I wouldn't think the air is coming from up forward, anything is possible though.

If we move the engines to the minimum of 20?" on center we loose slow speed handling but with long shafts the engines will be 5" deeper and the cav plate will be even with the bottom where it is supposed to be. That will require $10,000 worth of lower units etc.

Sal's Dad, we use the boat to access a remote cabin in SE Alaska. Usually hauling 4-6 people crab and shrimp pots, small freezer, lumber and a sometimes a small jet boat for river access. The deck is just over 12' it was supposed to be 14' but some issues showed up while designing and they left us out of the loop until it was too late to fix it. There is room on the stbd side for long lumber.

Manie B, masalai,

Found the Permatrim site, interesting but will it help? $300 beats the alternatives, may be worth a try.

Keep the ideas coming!

Many thanks to all!

TK

 

Pic of bow during construction.

 

If those things shown by manie, are "jack plates" then that is probably the best option - get the biggest ones... can be cut down - or I see you are in the buiseness? make some and try... I would not shift the engines - for both reasons you mentioned...

 

If there's room you can move the engines a bit more further apart. It gives more clearance and may help.. Anyway it doesn't cost anything to try

 

I reworked your photo and it realy seems that your motors could be too high, i still dont think they should be further apart just deeper with the permatrim plates, we had similar issues on Crazy Seal and we solved them the same way.

 

When the motors are trimmed all the way down the center of the cav plate is level with the bottom plate. With the V in the hull one side of it is above and one side is below the bottom plate. I can't lower the engines all the way but I'll raise the boat soon to start working on it. Boat hulls being so different there is no definite spec for how deep they should be mounted just guidelines and trial and error. The bummer is the boat builder drilled the holes at the top of the elongated mount holes so they can't be dropped without a bit of welding and drilling new holes.

Nice ride Manie B, custom one off or production cat? Displacement or planing hull?

TK

 

It is very important that ALL of the cavitation plate is below the hull else you will pick up air. Many years ago in my wild youth i had a boat (Raven) with one of the first 225 Mercury outboards that came into South Africa. The boat was used for barefoot water skiing and ski racing. I experimented with jacking plates that were screwed up / down by hand - very primitive. I would adjust it - go out - test - come back and reset. For my boat 20 mm below hull worked well. every boat is different. I also tried stainless props which was no better. You have big motors in salt water i guess 50 mm should be ok.

Redrill and lower the motors, no need to set them further apart.

Crayze Seal belongs to my school buddie, we have been drinking together since then and still get silly. She is a semi displacement cat which was built as a one off to fullfill his requirements. It is a party cruiser / lifestyle boat off note. The two 50hp Etecs are amazing. You will note that he fitted what we call dolfin fins onto the cavitation plates made of plastic HDPE. They work well.

 

I e-mailed pics and specs to the Permatrim dealer who is also a Yamaha dealer. They thought the props could be overloaded and recommended a 4 or 5 blade props, more surface area, and that the Permatrim would also help prevent air being pulled into the wheels. I'm not so sure about the overloading thing. Our neighbors at the cabin have a very similar boat twin 225's and 3 blade props that they use commercially. They haul huge loads without this problem.

It is very pronounced when the wheels unload, not gradual. If we try to go 8 or 10 knots while rolling in a 2'-3' chop as the boat rolls and one engine will rev then bite then the other lets go back and forth with the roll, very irritating. I'll see what they think about that.

I have an e-mail into the prop manufacturer to see what they think.

Manie, Thanks for all the input! I like the layout in that boat. Separate wheel house, cool. Bunks? or just the settee?

Currently leaning towards lowering the engines and Permatrims.

Extending hull plate closer to the cav plate? Would that help?

Keep the ideas coming folks, I need all the ideas, possible solutions and hopefully more real world experience to get an idea of what to do next.

Thanks to all!