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Old 29 June 2015, 09:22   #1
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Country: Hong Kong
Length: 10m +
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Outboard / Transom Splash

At about anything greater than idle speed, I get water splashing back from the engine up the transom. At higher speeds it sprays up over the engine.

I think I've been getting some good performance from my setup after some tweaking. At first I had a big ventilation problem until I added a hydrofoil and pumped the tubes to their max. Ventilation is now mostly gone except for if there is too much weight in the front.

Before installing the hydrofoil and before the ventilation would start, I could see most of the cavitation plate when cruising. Now with the hydrofoil, it's submerged under splash and I can't see it at all. The manufacturer/seller of the boat suggested I cut the transom and drop the engine to resolve the ventilation issue which I was very reluctant to do.

My transom is 15" and my engine from the top of the transom to the top of the cavitation plate is 16.5" so it is around the 1" below the hull/transom that is recommended for inflatables.

I've watched quite a few videos where the wake looks relatively "clean" without hardly any back splash and a thin wave or splash from the prop. Mine is not clean and the wave or splash from the prop is much thicker. We've tried all different trims and at the lowest it's the best for the ventilation issue. Could my engine be too low into the water? If it is, I'm afraid that if I raise it, my ventilation problem will be worse.

We only get our boat out about once a month if I'm lucky and it's not trailered so it's difficult for me to try several different scenerios. After we get it on the water, the family is ready to go and is loosing patience for me to play around and try to tweak the performance more.
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Old 29 June 2015, 11:53   #2
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A picture is worth a thousand words...

It's tough to diagnose what is happening at your transom without a picture of the outboard's cavitation plate as it lines up at the bottom of the transom.
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Old 29 June 2015, 11:57   #3
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Country: UK - England
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Boat name: Bombard, Y-162
Make: Aerotec 380, Y-Class
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Read through the below carefully, Loco really did a great post here:

ABC Sib-Rib Installation Guidelines.-
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Old 29 June 2015, 12:23   #4
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Doing some more searching I've read where the Tohatsu/Mercury engines with the square cowl have the worst back splashing. From additional videos I've watched where the engine was sitting too low and then corrected, I think my question and concerns were answered in that my engine sits too low on the transom.

I attached two images that I found that resemble my condition. One that shows what it is like when cruising and the other for WOT. Only difference is that the water splashes more forward and on top of the engine.

If my cavitation plate is 1-1/2" below the hull at the transom, then should I start with a 1/2 or 1 inch trim piece to experiment?

Guess I could create both but it would be harder for us to swap them out after we've launched. The boat ramps we have to use here are busier than most highways.
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Old 29 June 2015, 12:27   #5
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Quote:
Originally Posted by Max... View Post
Read through the below carefully, Loco really did a great post here:

ABC Sib-Rib Installation Guidelines.-
Funny, I had just got done reading the same posts but on a different forum that came up in a search;
ABC Sib-Rib Installation Guidelines.- Page: 1 - iboats Boating Forums | 644353

It was reading this thread that leads me to believe it's sitting too low. Just not sure how much. Guess I'm just going to have to play with it over time.

Testing isn't easy for us. The boat launch is extremely busy and we're usually frantic trying to get in line to get launched. It's been twice now that I've forgotten my camera.

Thankfully we've had good trips in the boat. Would be nice to get it more in line if possible. My fear is that raising it would bring back the ventilation issues again and make for a bad trip with my family especially since it's rare for us to get out. Would be great if we could remove the shim while out on the water if it's not working well but I don't know how difficult that would be.
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Old 29 June 2015, 13:03   #6
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Just take some pre-made wooden shims with you (say 1/2", 1", 1.5") and slip them under the engine mount trying each with some speed/splash runs. *

As per Loco's topic ALL SIB/engine combos need setting up this way. Additionally you need to take a Tinytach or Chinese copy and check the best trim position for the engine and that the prop gets you to within a few hundred RPM's of max at WOT.

*Even with the height dialled in you will get some splashing, my Merc is the same.
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Old 29 June 2015, 13:51   #7
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Sounds good. Yes, I'll make the different shims and give them a try. They can be loose for testing and don't need to be screwed down?

Had a similar tach hooked up this last time but was aggravated after trying to get it to read the rpms for a 4-stroke before it quit working. Was going to return it and try a different one but the seller is going to send me a new one without a return. Not exactly what I wanted but oh well, will give it another shot.

At WOT and at about 17-18 mph, before the tach stopped working and when I think I had it on the right setting, I read about 4700 rpm which should mean my prop is getting proper grip with minimal slip. That reading sounded right compared to when I listen to the full range with throttle only. Am thinking I didn't get a higher rpm because of the potential drag the lower unit is causing by sitting too low but I do know the engine is also under powered which is also understood and fine by us.

I'll now have to wait until mid to later July to try it again...
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