C21 boat propeller pulls air when turning

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bartmatthijs
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C21 boat propeller pulls air when turning

Post by bartmatthijs »

Dear,

on my new c21 i put a honda 135 hp, long tail... runs great and gets about 60 kilometers per hour.
We have also waterskied and it is going well.

2 problems :

1) When turning at high speed the boat propeller takes air and the engine goes into high rpm and loses its power

2) The boat only comes to a flat position at 35 kilometers per hour . Does anyone have experience how we can adjust this, the engine is completely down

I'm adding some pictures of the boat to give you an idea

Bart
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fallguy1000
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Re: C21 boat propeller pulls air when turning

Post by fallguy1000 »

The engine is too high and this causes cavitation during turns.

The boat is bows high until you are going 21 mph? This is either due to too much weight aft or because the engine angle is too far out on the bottom.

Show engine down pictures on a trailer, side views
My boat build is here -------->

viewtopic.php?f=12&t=62495

jacquesmm
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Re: C21 boat propeller pulls air when turning

Post by jacquesmm »

As Fallguy says: probably engine too high. Please post pictures of the engine on the transom, pics taken on the trailer, side view.

BTW, where in Belgium are those pictures taken? I see platbodem boats in the back. Is that in Holland?
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bartmatthijs
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Re: C21 boat propeller pulls air when turning

Post by bartmatthijs »

Not Belgium … Holland, 9291 MB Kollum
I send some pictures soon

TomW1
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Re: C21 boat propeller pulls air when turning

Post by TomW1 »

Beautiful boat agree with the above. The ventilation plate should be even with the keel if it is not now.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

bartmatthijs
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Re: C21 boat propeller pulls air when turning

Post by bartmatthijs »

I think the situation is very clear after your comments,… Many thanks !
I drew a line equal to the bottom of the boat so that it can be clearly seen that the anti-ventilation plate is 15 cm too high.
The motor must be placed 15 cm lower !

Can I shorten the transom by 15 cm without affecting the strength and structure of the boat ?
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jacquesmm
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Re: C21 boat propeller pulls air when turning

Post by jacquesmm »

Yes, that is the problem. The transom is designed for a 25' shaft (635 mm), with a safety margin for adjustment, the plans show 620 mm. Yours look like it is 5" (12.5 cm) too high for that engine. Your engine must be 510 mm. Metric or imperial, they are the same and a 20"/510 mm is very common in that size engine.
You can cut the transom down by 12 or 13 cm. No strength problem but the steering and fuel lines may hit the bottom when tilted. You may have to move the splash well bottom too.
It is much easier to do than to raise the transom.
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OneWayTraffic
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Re: C21 boat propeller pulls air when turning

Post by OneWayTraffic »

Why not get a longer shaft engine?

jacquesmm
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Re: C21 boat propeller pulls air when turning

Post by jacquesmm »

If he can trade the engine for a longer shaft, that would be ideal.
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bklake
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Re: C21 boat propeller pulls air when turning

Post by bklake »

These motors have several mounting holes, no? Lower the motor to it's lowest point and see if that is enough. Also look at adjustable jack plates before cutting into the wood.

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