Trophy on Plane

[Kwilu]

I have a 1992 Bayliner Trophy, 1992, 20' Cuddy (model 2059) with 175 Hp engine and Mercruiser Alpha 1, Gen II stern drive (I believe that the gear ratio is 1.81). It weighs roughly 3400 pounds dry and roughly 4400 pounds loaded. It currently has a 14.5 x 19 prop and a Davis doel-fin hydrofoil. It also has trim tabs. 8' beam.

My boat has an extremely difficult time planing. At 1500 rpm the stern drops and produces a large wave. As I throttle up to almost full WOT (4400 rpm) the boat may plane (come out of the hole) and may not. In the ocean, with waves, I can drop in and out of plane while remaining at full WOT. It does help a little shifting weight forward but that does not seem to be the key issue. When I do plane the boat hits around 25 mph which seems low for the rpm and pitch (or at least it means that I have very high percentage slip).

I definitely plan on changing to a 4 blade prop, but I am not sure that will really take care of the problem. Any ideas or thoughts to checkout. Would a smaller pitch help in my situation?

Also, the original specs for the boat stated that the original prop was a 16 x 16, but I am doubtful about this. A 16 inch dia prop would hit the cavitation plate.

 

[alldodge]

That is a lot of weight for a 4 cyl to move, even if the motor is in top shape. If the motor has not been touched (rebuilt) then I think it may also be tired. Adding another blade to the prop will take more power away from the motor. More blades more hp needed to turn it.

I would suggest doing a compression and leak down test to check the health of the motor.

 

[Maclin]

I can't tell what engine you have based on the description. Can only guess, from the Year and the HP and Ratio, that it is a 4.3L. I would try a 17pitch 3blade propeller, you are struggling mightily with the 19pitch. If you want to jump to a 4blade, then go with 16pitch. In all scenarios I would remove the fin until you can isolate the problem to prop pitch or general engine health.

You probably need to give your engine a general diagnostic, starting with compression, to see if it is healthy enough to power that boat. Throwing a prop at it, if it is not up to par, will only mask the problems.

Basically same advice if it is a 4clinder 3.0 with 135 HP, prop is too much pitch right now.

 

[Maclin]

Manners... Welcome Aboard!

Mods, seems like this should be moved to the Prop section?

 

[Kwilu]

Thanks for the input so far. Just to clarify the engine is a 4.3 l, 175 hp, 6 cylinder. My thinking, which could be incorrect, is that the boat does not falter from hitting the 4400 rpm's (and does not bounce around) and so I would think that I am supplying sufficient HP to rotate the prop at (4400 / 1.81) 2431 rpm's.

I will run a compression check to validate cylinder integrity, but based on how the boat runs/sounds I do not think it is a HP problem. It seems that I am not getting the thrust and/or lift I need to keep enough speed to stay on plane.

 

[Maclin]

Ok, assuming the tachometer is accurate, and the speed is from GPS or otherwise verified, the slip is 40% or more, that is untenable. With that much slip, if the 19pitch suddenly dug in then your rpms would drop a lot I bet, hypothetically. In my view, a 17pitch stainless in a good brand would grab better and hold it on plane easier, and be much safer if you have to skeedaddle to beat a storm in. Your first feeling that a 4blade would be better is probably correct also, in a 16pitch. In my opinion, since you cannot go above the 4400 now with that much slip that you need to stay lower in pitch. My opinion.

When you say the original specs show a 16x16, where exactly are you seeing that?

 

[Kwilu]

Thanks Maclin, yes the speed is from my GPS (good point) and yes the slip is very high. What you are saying makes sense to me from a physics perspective. Ultimately I really do not care about top end speed (ie hitting 40 mph plus), but would like to cruise on plane in a reasonable fashion. A 17 pitch would not be bad and if it actually grabbed the water and created thrust I would be in better shape.

I saw the spec for the 16 x 16 prop in the following PDF on page 19 http://www.trophyboatparts.com/1992/2059FD92.pdf

I am interested in the fact that you quickly stated "remove the fin until the problem is isolated". I have my own feeling about the Doel-Fin from an experience years ago, but I am curious as to your reasoning. Is there a pro or con to leaving this on? Is there a better solution?

Thanks

 

[Maclin]

I am mentioning the Doel removal as a method of going back to basics and starting the troubleshooting from there. I do think tabs do a much better job of affecting the hull performance positively when compared to a wing out on the drive. Smart Tabs or other brand may help in your situation, sorry I did not think of that in previous posts.

I read that PDF, interesting and lots of great info on your boat!

I think that prop recommendation stands out, the 16pitch part anyway. Same pitch recommended for 3.0 and 4.3, indicating the different ratios between them. Bayliner knows how heavy it is, and how the hull reacts, so that 16pitch from them means a lot. I would be tempted to go back to that, a 16pitch, remove the Doel and see how differently it behaves.

I recently went back to a factory recommended prop on mine, as I had put on something different and got bad results. Now I have a predictable and good base performance again.