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Setting the right motor depth for a hydroplane

S

SaltyScot

Regular Contributor
I'm running a 76 Johnson 35hp and an 80 Evinrude 35 hp and I need to know the right level to have the prop on my hydro to keep the water flow uninterrupted. Does it differ from the usual cavitation plate level with the bottom of the boat? I tried that and I believe I was losing flow on tight turns under deceleration to the tune of an overheated Evinrude. Both these engines are 22" from the clamps to the cavitation plate and I made the transom 22" to accommodate them specifically. Boat is 11' long and 76" wide at the transom. It has very little bow rise and draft when on plane. Should I go deeper with the motor or add a low point pickup like on a tunnel hull racer? Maybe add some weight in the back or a 50hp Mercury at 60lbs heavier? :confused: Please help before I'm buying head gaskets in bulk. :eek: Thanks.

Stephen
 
My gosh, I just looked at the video my son took of me in the boat and it looks like it's drafting so little that the water pump might not even be in the water. Despite the fact that I weigh an eighth of a ton! I can't believe how buoyant this thing is. I think I have to trim some transom. A little bit of bow rise would be nice too.

Once again, the cavitation plate is even with the bottom of the boat.
 
I made the mistake of getting a new iphone recently. I cannot figure out how to downsize the photos to fit here. They are all 6-12 MB. It was easy on my S8. Everything seemed more intuitive.

But in looking at it, I can't see how it could pump water. Just like you schooled me in my other thread, there needs to be 6-8" of water above the impeller and there sure isn't even close. Every time I would start it up, it would pump fine. But in the turbulence of the turns and decels, it can't seem to handle at this height.
 
Nope-----It only needs water above the impeller when static.-------Once pump is primed and the boat is MOVING at speed it is a different story !
 
Who engineered the boat? What do they say? Just looking....that's not deep enough....if it's not cavitating, it's a miracle. On step....in a corner....you WILL LOSS PRIME to pump....I think you have to do something before you burn up all your motors....it's gonna be more than head gaskets, my friend.
 
Thanks Tim. The boat was originally engineered by naval architect William Jackson in the 50's. This longer version was created much more recently. It's just enlarged 20% in every direction. The XL version was re-engineered by muskokaseaflea.ca maybe 10 years ago. There's about 6 or 7 videos of these boats on youtube. One of them rides very flat like mine and has a short shaft motor with a large bow mounted gas tank. The plans call for a 20" transom for use with a long shaft motor at 10'6" length. I thought since my motor has a 22" shaft that I should add an extra 2" to the transom. I made my boat 6" longer than the plans to use a 50hp outboard, considering my size and to the possibility of a second passenger.

So if I keep this length shaft motor, I should cut at least 2" off the transom? How about a heavier motor? One guy has a 90's 50hp Johnson with a huge jack plate and power tilt and trim on his. Probably like 100 lbs more than my 35hp. And his is under-length at only 9'6" long and has cavitation problems. I probably should have just went with a Dillon Tunnel hull(and followed the plans exactly), but I wanted a two-seater.
 
Okay....good info. It's certainly your call but for me I would rather keep a lighter motor and shave down the transom until it works. The 35 is a very potent motor for its weight.
 
timguy said:
Okay....good info. It's certainly your call but for me I would rather keep a lighter motor and shave down the transom until it works. The 35 is a very potent motor for its weight.
Click to expand...

Thanks for the continued input sir. I will be shaving this transom down shortly.
 
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