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Old 17-01-2018, 00:39   #1
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Trouble with my outboard height?

I've had this dinghy for a few years now, it's going well, but I've always taken it easy because it had an issue. At anything above half throttle, the water would funnel up and enter the boat right where the engine mounts... it would literally flow in at a couple of litres a second, up the transom and over and into the cockpit.

I raised it with a piece of timber as you can see in the pic below, however from perhaps 2/3 throttle and up, I get a massive rooster-tail on both sides, as you can see from the pic.

What do you think... do I need to raise it further?

It happens when it's lightly or heavily loaded. It's 30hp and 4.3 metres.



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Old 17-01-2018, 04:24   #2
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Re: Trouble with my outboard height?

How much does the outboard weigh? What were the dinghy manufacturer's recommended shaft length, HP and motor weights?

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Old 17-01-2018, 04:28   #3
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Re: Trouble with my outboard height?

Typically you want the cavitation plate on the outboard to be even with the bottom of the boat, if a v'hull the bottom of the 'v'...
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Old 17-01-2018, 05:51   #4
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Trouble with my outboard height?

To expand a little, when on plane you should be able to look down and see the top of the anti cav plate, the top should be barely out of the water, but the bottom of the plate riding on the water.
If your motor is too deep, you will be amazed at the performance increase when you get it right, it may be so much that you need to reprop.
Raising the motor can cut drag enormously, it’s why bass boats have jackplates.
Of course get it too high and the prop ventilates at the drop of a hat and anytime you try to turn
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Old 17-01-2018, 06:25   #5
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Re: Trouble with my outboard height?

Quote:
Originally Posted by ausaviator View Post
What do you think... do I need to raise it further?
Yes so that the anti ventilation plate is an inch above the bottom of the v hull for a planning boat.

This will leave a gap between the top of the cut out transom and the underside of the engine saddle (bit that bolts to the transom). Fill this in with a piece of hyperlon to keep the water splashes out or if you stop quickly when the stern wave catches you up.



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Old 17-01-2018, 06:42   #6
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Re: Trouble with my outboard height?

Be very, very careful raising the engine, once you find the sweet spot, consider bolting the engine through the transom, cause if the engine isn’t sitting on the transom, it can easily come loose and come off.
I learned this the hard way
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Old 17-01-2018, 08:22   #7
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Re: Trouble with my outboard height?

Are you sure it's an engine issue? Anything attached to the transom that may hang lower than the bottom of the dinghy may be part of the problem. The depth-finder on my boat creates a rooster tail when at plane, I notice you've got dolly brackets attached to the transom, could they be the issue?
My experience with boats are: transom depth determines whether the boat will accept a long-shaft or short-shaft motor. Too short of a shaft and you will most-likely experience cavitation issues when at plane or trying to plane. It appears you may have too long a shaft for your dinghy.
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Old 17-01-2018, 08:26   #8
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Re: Trouble with my outboard height?

Looking at the picture and rooster tail, you might be suffering from thrust angle. Real boats have trim to deal with this. With our dinghies the optimum angle putting along, and on plane, can be significantly different. It looks to me like the motor's thrust is up (rooster tail) pushing the transom down and probably the bow up. I would try making the motor a bit more vertical.
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Old 17-01-2018, 08:36   #9
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pirate Re: Trouble with my outboard height?

Down size.. if you can only use less than 50% of throttle to make it flood at dinghy manufacturers transom height your over powered.
A 15/20hp will move you just as well.. and safer.
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Old 17-01-2018, 08:44   #10
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Re: Trouble with my outboard height?

Do you have a “long shaft” engine? Is the tilt adjusted correctly? When all is adjusted correctly there should be no rooster tail. You may need a jack plate to adjust it properly.
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Old 17-01-2018, 08:57   #11
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Re: Trouble with my outboard height?

You could be experiencing chine walk, when your cavitation platr on engine is lower than the bottom of your hull. plate needs to be the height of the bottom of your transom.
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Old 17-01-2018, 08:58   #12
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Re: Trouble with my outboard height?

I have a Tohatsu 9.8 on a Highfield 310 Classic. The transom height on the Highfield is 15" (the industry standard for a short shaft outboard). The Tohatsu measures 17" and is stated as that in the owners manual. I can't explain the discrepancy but I know if I don't raise the motor as high as possible (about 1"), water will churn over the transom when on plane regardless of the trim angle.
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Old 17-01-2018, 09:10   #13
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Re: Trouble with my outboard height?

I agree with most of the previous posts. Generally, engines come with 15", 20", or 25" shaft lengths. Transoms, too, are intended for those engines. I have an aluminum hull 3.8m RIB and it has a 15" transom height, so I am limited to use engines that have short shafts (in my case, a 25HP Suzuki). Sooo, the first thing I would determine is if you have a long shaft engine on a short shaft transom. I know you've thought of this, but that would explain a lot.

I'd also experiment as far "up" as you can go without endangering your outboard. The challenge is that you don't want to temporarily bolt it in the wrong place. But would it be possible to shim the engine up further, with tie downs to keep the engine from working further up, and see if the situation improves?

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Old 17-01-2018, 09:29   #14
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Re: Trouble with my outboard height?

Interestingly, we had that problem with 250 HP Hondas on V bottom boats. The engine was installed exactly like the other manufacturer's 250 hp engines were. (Usually Etec or Suzuki) I cant remember what the resolution was as it's been 10 years, but it was either tilt adjustment or change in mounting height.
Our outboards were mounted with cav plate at bottom of boat to 1" below. The "bottom" of the boat being: "the bottom directly in front of the prop shaft". So on a dual engine boat it was well above the "keel" bottom.
I wonder if the 30 HP is just too deep for that boat?
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Old 17-01-2018, 11:35   #15
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Re: Trouble with my outboard height?

I mount mine one inch below the bottom of the boat. The anti-cavitation plate has to be in clean relatively undisturbed water to work properly, on turns, and chop, etc. Have you talked to the dinghy and engine manufactures? Engine size sounds high for a dink; what's the recommended HP?
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