Motor height

[ddywz]

When I first put my Honda BF50D on my zodiac future MKIII was debating with myself for the motor height. There's lots of conflicting information out there. Even the manual says that the ventilation plate should be on same line with bottom of the boat. Since silicone and drilling was involved I wanted to possibly do this only once but that was not the case. I purchased all manuals available and read lots of materials for installing the outboard and finally decided to position the ventilation plate on straight line with the keel of the boat. The engine is a long shaft 20" and in order to do so I had to use the 3rd hole (upper mount on the engine side) from the top and at the same time this hole was 2.1 inch from the top of transom. I drilled the transom with a template that the Honda dealer guy was nice enough to let me borrow and use it. After sealing everything and let it dry for 24 hours I took the boat for its first test. After trolling on 850-1K rpm for about 2-3 hours to do the initial burn-in tried to increase the speed in bursts. All was good until you try to go over 2.5k rpm. The boat started to plane and so the engine lifts up with it. I tried 4k rpm and its speed dropped as it was sucking air from the plate. I dropped the speed immediately for protecting the engine and tried not to pass 2k rpm. I trimmed it too low so the plate could remain on water and doing so I was able to go very fast and could see 5k rpm on the tachometer. I felt this was not right though. After looking this up I read that some recommend the cavitation plate to be 1.5 to 2 inch below bottom of the boat. Today I removed the engine back and dropped down to its 1st hole from the top. Now the ventilation plate is 1.5 inch below the keel of the boat. Will give it a try now and see how it performs. My hope is the cavitation plate will stay under water when the boat planes. Anyone else had similar experience?

 

[Sea Rider]

Re: Motor height

You are right about existing too much conflicting personl information going on here, posters will recommend what's stated on owner's manuals, some own pure theory & myths, others achieved water tests end results.

Some universal truth :

Engine manufactures doesn't care a bit which boat type you'll be installing their engines on.

Engine brands although having same S, L, XL sizes, all have different lowr leg distances, shapes, configurations.

Boat builders doens't care which brand engine you'll be using. Just states max HP allowed.

Boat builders manufactures boats with different sizes, transom heights and hull shapes.

Dealers selling spare engine and boats doesn't care a thing about their individual performance.

Final boater is left all alone to deal with 2 different world products that works together.

Bottomline : The best engine/transom match height for top combo performance will only be achieved under water trial and error as this is not a excact rocket science, too much variables to deal with and take into account such as : HP, engine leg shape, transom height, trim, hull shape & water conditions.

Happy Boating

 

[deejaycee_2000]

Re: Motor height

The ideal height on an inflatable would be to alight the centre of the high jackers to the centre of the prop ....

 

[lwilde13]

Re: Motor height

ddywz did you ever figure out the solution to your problem? My 10' inflatable is doing the same thing. I idle at about 3mph and when i start applying throttle the boat tips back and when i hit about 7-8 mph the boat starts to lift up and plane. However, as soon as the boat planes or levels out the prop spins, the rpm's shoot up and I end up stopping until the prop catches again at about 3 mph. Please let me know if adjusting your engine height helped. I believe I need to adjust my motor down an inch or two on the transom.

 

[deejaycee_2000]

Re: Motor height

With inflatables the engine height is different as on normal boats ..... here is a pic on what I meant about the centre of the prop lining up with the centre of the highjackers on an inflatable ....

 

[lwilde13]

Re: Motor height

My boat doesnt have those cool highjackers. its just a flat bottom boat. Is there any kind of rule of thumb for an inflatable like mine on prop height to prevent cavitation? like the prop should be at least 2 inches below the bottom of the boat at its highest point? thanks for the help fellas! This is my little wave jumper

 

[lwilde13]

Re: Motor height

shoot, still learning how to use this forum

 

[Sea Rider]

Re: Motor height

Hi lwilde13,

Before week end should have finished writing my personal ABC Boating Guideline, includes tech matters and pics for full understanding, a very easy to follow guideline for new boaters that wants to optimize their boat/engine combo to achieve top performance starting day 1. Once it's ready will post.

Happy Boating

 

[deejaycee_2000]

Re: Motor height

yes I would say about 2 inches below the lowest point .....

 

[ddywz]

Re: Motor height

ddywz did you ever figure out the solution to your problem? My 10' inflatable is doing the same thing. I idle at about 3mph and when i start applying throttle the boat tips back and when i hit about 7-8 mph the boat starts to lift up and plane. However, as soon as the boat planes or levels out the prop spins, the rpm's shoot up and I end up stopping until the prop catches again at about 3 mph. Please let me know if adjusting your engine height helped. I believe I need to adjust my motor down an inch or two on the transom.

Sorry for my delay here... I actually did drop it two holes down (Honda engine-2 holes=35mm=1 3/8 inch) This made a big difference. Now even if it planes, boat does not lose speed and ventilation plate remains under water. It is much better now as at least the plate remains under water so the impeller continuously provide water for cooling the engine. I'm good for now although am trying to fix the trim alignment. If I keep the trim to 0 (according to trim/tilt Honda meter I installed on console) and start speeding up, the bow starts raising up a lot before all boat goes back parallel with water surface. Trimming to minus - side (forcing the bow down) helps reach the parallel position of the boat faster and at smaller speeds but I noticed that this affects the max speed you can reach. After the boat planes I start trimming back to + side trying to reach 0 level and the boat remains planed as long as you don't drop the speed and actually speed increases a bit as you keep trimming on + side trying to reach the 0 level. No, I don't have the weight all accumulated on the rear and actually is spread along the boat. I mounted the aluminum benches and sit myself almost on middle of the boat and have another person in front of me and the bow still raises high when you start gaining speed. Ohh well, not a big deal and maybe this is normal for inflatables but I'm trying to correct as much as I can. I do have to admit though that the 50Hp motor I put in is the MAX recommended from the manufacturer of the boat and this brings 220lb on its transom. Getting back to the topic, remember when we say the ventilation plate should remain under water regardless of your engine throttle this is with the assumption that the water is totally calm and no tides/waves around the boat. It is quite normal on bouncy waters for the plate to temporarily come out of the water and this is expected and I would say is quite ok. I suggest you try to lower it a bit maybe 1 inch or so if you can and see if it will make a difference. It would have been better to have the motor height adjustable from the console but this task is not an easy one to accomplish mechanically especially not worth for inflatables. This is something that only you can fix by adjusting and testing it until you find the sweet spot for the motor.