after upgrading to a a15hp outboard my boat doesnt plane well

Hello guys, I am looking for some help/advice.
(English isn't my first language but I am trying my best.)

I am a young guy and I got my dad's old boat from when he was little, it's a super light fully fiberglass boat. Here it is:




I've had a 5hp Tohatsu behind it for the past 3 years and for a 5hp, it ran amazing and was really fast. It came loose from the water so there was very little drag. You can see that the last 40cm was the only wide part of the boat that was in the water; with a little wave the entire front came loose. That was what made my boat way faster then all the rubber dingys my friends have.



But i got an really old 15hp (probably 10hp left in that thing); it's a 1976 15hp Evinrude. But my boat doesn't plane as nice. When I have the trim at the 1 or 2 (from the bottom setting) the nose dives (even when sitting all the way back) in to the water way more then before. At higher trim levels the boat starts jumping and I am forced to move further to the front. And I am back to nothing. I have my fuel tank at the back of the boat so that shouldn't be a problem. I don't know what to do? Do you guys have a solution?
Thanks in advance!

 

Welcome to the Forum Ole Bos.

You mentioned that the boat would run very well (and very fast) with the 5 hp Tohatsu - if this engine is still operational, maybe you should just carry on using it instead of the 'new' (to you) 15 hp Evinrude?

 

with the 5hp my max speed was 30kmph, now it is 39 kmph. so it is faster! but i think it could go way faster and glide way smoother

 

Why do you want to go 'way faster'?
Surely a speed of 30 km/h is plenty fast enough for a dinghy like this?
And why do you think that it will 'glide way smoother' with more power?
It will only 'glide smoothly' in flat calm water - will you be using the boat on your inland waterways, or the more open North Sea?

 

after 3 years i am a bit bored of the speed, i just want it to glide with the same smoothness as before, now it is just worse with more power there is more power to get it out of the water. i may be wrong tho? i only use the boat in inland waterways!

 

Unless the Evinrude is completely worn out it should make the boat go dangerously fast. and at 35 km/hr that is already pretty fast for such a small boat.

The additional weight of the larger motor may be a problem. The propeller that you are using on the larger motor may be holding you back. Depends on the diameter and pitch of the prop and also whether the motor has the remaining power to turn a prop with more pitch. Is the prop three or two bladed? Are the leading edges of the prop blades in good condition? Are any of the blades bent?

The cavitation plate (the horizontal plate that is just over the top of the prop) should be positioned so that it is perhaps 3 to 4cm below the bottom of the boat. You could raise it so that it is that it is even with the bottom of the boat and you might gain a little bit of speed. To raise it, you can use what are called "rev sticks". These are simple pieces of wood under the clamp bracket that raises the plate to the most favorable position. Beware! Do not put too many of those thin sticks under the clamp bracket. However if you have that plate too high it will cause the prop to ventilate when turning the boat. Measure the height of the transom. They are usually about 38 cm high. Measure the length of the leg of the motor. The distance from the bottom of the clamps to the cavitation plate is ordinarily about 42 cm more or less

In the U.S. we still use the primitive imperial system of measure and the outboards are described as 17 inch, 21 inch, and sometimes longer. Measure the distances to be sure that you have a so called "short shaft" outboard motor. Or simply compare it with the Tohatsu. If the shaft is too long the boat will not work as well as you wish it to. I expect that you already know that the "tilt angle" controls the way the boat behaves. If the leg is tilted too far outward, the nose of the boat will tend to rise and may cause porpoising. If it is too close to the transom it will tend to drive the nose of the boat down. The trick is to find the most appropriate angle, which is easily adjustable.

 

Is that rot in the top of the transom?
A bigger, heavier, and stronger motor may not be a good idea for a very lightly built old boat that already performs quite well with a low horsepower motor.
Your quest for speed may require an actual speedboat to safely operate at the speeds you’re looking for.

 

There isn't a solution. That hull shape simply wasn't designed to carry that weight that fast, and it won't do it comfortably. If you run the old Evinrude 15, which weighs about 105 pounds if memory serves, at 15 knots, how does it run? Probably fine, with you sitting on the rear bench using a tiller extension running trimmed 1 hole up.

Realize that when that boat was built, there probably wasn't an outboard motor made bigger than 25hp.

 

Most likely this is a prop problem. The old Evinrude is not achieving wot.

But if the transom is rotten, thank God your propped wrong.

Another possibility is the new motor leg might be a 20” and this is a 15” transom. This could be the cause of the bow dropping as you basically have a large brake on the back. I see messabout mentions as well… this was my 1st thought

 

i agree with 'fallguy'. check to see if your new 15hp has a 20" leg/shaft, compared to your old 5hp (maybe a 15" leg/shaft). this would make a huge difference. if shaft lengths of both motors are the same, then could be propeller issue. (what are your 2 props used for 5hp and 15hp motors?)

 

@ole bos often the extra power and mass of a bigger outboard will cause issues with trim and longitudinal stability in these situations. Generally increasing the area of the anti-ventilation plate with a foil shaped wing will improve it considerably, this type of thing.
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