motor height on Valiant D380 boat

[Gonga]

Hey everyone,

I recently bought a Valiant D380 RIB boat along with a Yamaha F20 outboard motor. Since this is my first boat, you can say that I do not know a lot about boats. Because of this I had to read my motor manual thoroughly. I noticed something when I mounted the motor on the boat. It sits too low. The anti-cavitation plate is around 2" lower than the lowest V point of the boat. I've driven the boat only once and I noticed that on lower speeds it does not run well (like it doesn't have power), and on higher speeds it tends to push the nose downwards. The tilt angle was on the lowest point. I cannot mount the engine higher(maybe just a 1/2"), because the front mounting screws can't go any higher on the transom.

My question is this: Can I mount the motor only with the mounting holes on the transom? Will it hold? Because otherwise I will have to do some major modification on the transom in order to mount the motor higher?

 

[Sea Rider]

Re: motor height on Valiant D380 boat

Measure the distance from where engine sits top on transom vertically to rear V keel, if it's around 40 cm is for a short shaft engine. Try trimming the engine at second/thrid hole out from transom and distribute your weight, mate's weight near middle to bow.

What brand is your 20 HP model motor ? What is stated on owner's manuals is a refference about min and max transom heights to match cav plate distances in order for the sib/rib/engine combo to work as expected. But for best performance, it's a trial & error issue with final transom height modification.

Happy Boating

 

[Gonga]

Re: motor height on Valiant D380 boat

The motor brand is a Yamaha as I stated previously. The motor is short shafted. The boat requires a short shafted motor. But still, the anti-cavitation plate is 2" lower.

Here are some pics:

 

[Sea Rider]

Re: motor height on Valiant D380 boat

It's normal for the cav plate to be between 1" to 2" under the keel measured on a horizontal imaginary line, don't confuse small upper plate, the one facing drain water valve with lower anticav plate. Your actual tech condition matches exacly my same 420 rib condition, both ribs have 42 diam tubes.

For your peace of mind perform this simple water test. Inflate rib to at least 3.0 psi, start testing rib/engine combo at second hole out transom, have a non skinny guy sit at front bow deck, sit on transom deck next to engine, fully throttle engine, once rib is on plane at a straight course, check back engine and see where is the water flow passing by with respect to tail, if passing at red area will have splashing issues, ok if passing by green area, the best position is water flow passing slightly under small plate (green dotted line) perform this test on calm flat no wind waters if possible. Check pics,

Test same using third hole out. Use the hole that gives tubes and cav plate parallel to surface water condition once on plane. Use your kill switch lanyard secured on you and grab with other hand rope tie if available, just in case. My thrust rod is fixed to second hole out from transom and rib is properly weight distributed as in first pic to achieve best wot speed performance and inmaculate tight turns with no cav issues whatsoever.

Happy Boating

 

[Gonga]

Re: motor height on Valiant D380 boat

Thanks for the pics, they helped. I'm not confusing the plates, all I'm saying is that the cav plate is 1" lower than the maximum allowed in the motor manual. Also, do you think that it will help if I try different trim angles?

 

[Sea Rider]

Re: motor height on Valiant D380 boat

Nothing is cast in stone, owners manuals states ideal min, max cav plate heights as parameters to have in mind but it's a water trial & error condition. Don't be lazy, perform the cav test as described and check your actual water flow if in pursuit of best rib/engine performance. Definitely distributing your weight correctly and varying the trim angle will vary the sib overall planning condition and nose behaviour. Matching first pic condition, you are onto something, welcome on board...

Happy Boating

 

[Gonga]

Re: motor height on Valiant D380 boat

Ok, I will do what you told me, and then I will post again. Thanks again @Sea Rider.