Hydrofoil

[mulliganmagic]

My grandfather owns a 1983 Norris Craft fish and ski boat. Even though it doesn't belong to me I use it with him every time that he goes to the lake. But the boat has a problem with the ride. Even though it is a fish and ski boat it actually has the same hull as the same length (17.5 ft) bass boat that Norris Craft builds (the only difference is more seats in the bow). It has a 1986 150 hp Mercury Black Max XR2. He runs a 23p Quicksilver prop and it hits about 5500-5800 RPM at WOT depending on the load. This boat is a serious machine and it will run close to 70 mph I would say (speedomitor doesn't work correctly). The boat has a good hole shot of about 4-5 seconds, and yes the bow does shoot the moon. But now that you know the set up our problem is with porposing. We don't run the boat wide open all the time we usually run about 3000-3500 RPM to cruse. However, at this speed the boat seems to porpose a little (one reason for this is because these boats are very light in the front). I guess my main question is would a hydrofoil fix the porposing problem on a motor this big. I have suggested smart tabs for this boat but he said that they are too expensive for that small problem. Also, I read on the forums that for the foil to work right it has to be trimmed up, but right now we have to leave the engine trimmed all the way down until we push it up to at least 4500 rpm or so because if we trim it up it porposes much worse than before. Sorry if this is confusing but I hope that you can help with our question.

 

[JB]

Re: Hydrofoil

Outboard Non-repair.

 

[fireman57]

Re: Hydrofoil

I wouldn't consider porpoising at close to 70mph a small problem with passenger aboard and other boaters around. Even at 50mph it is unsafe.

 

[mulliganmagic]

Re: Hydrofoil

Sorry I didn't make that clear, but the boat does not porpoise at 70 mph it only porpoises at about 30 mph. We don't like to run 70 mph all the time because it uses too much gas. I was asking about the hydrofoil to see if it would help improve the ride of our boat at cruse speed 30 mph at about 3000-3500 RPM. This is when the boat porpoises, and after you push it up to 4000 RPM it stops porpoising and gains more speed proabably 40-45 mph.

 

[CATransplant]

Re: Hydrofoil

Sounds like 30mph isn't the sweet spot for this boat. It's planing, but not at an ideal level. When you speed up to 40-45, the porpoising stops, you said, and doesn't return, even at 70 mph (a scary speed on the water for me).<br /><br />I don't know that boat design, but it sounds to me like you're not really fully planed out at 30 mph. You could be in an intermediate zone for the boat, with a little too much water under the hull. How to fix this? I don't know.<br /><br />Trim might help, but it may just be part of the boat's design. It may just not like running at that speed. Will the hydrofoil work? Can't say. It will change the parameters, but might just shift the bad behavior to a different speed.