question about doel fins

Was having a problem with the prop slipping and tried some doel fins to compensate. The fins solved the slippage problem but I have lost about 3mph on the top end. When the boat is on plane the fins ride just under the water. I thought they were supposed to skim just above the water line. Is this correct? Im not sure but Im thinking that my motor height should be a little lower without the fins(obviously) or a little higher with the fins. Does this sound like the correct remedy? Also which would give me more overall performance with or without the fins? Have tried different tilt settings but the optimum setting is where my prop will occassionaly slip. thanks

Re: question about doel fins

If you give us the details of your boat/motor,prop condition & size(on barrel or under the prop nut),speed and rpm we can tell better where your at.
For the fin to do the comeplete job it needs to be in the water but it will lose speed.Get rid of the fin and get a prop that resists venting a turning point
Hustler or Rascal depending on the size of your motor.The prop will out perform your present prop and resist venting.It may even allow the motor to be higher.The higher the motor the more efficient.

Re: question about doel fins

Since you talk about raising and lowering the motor we'll assume you have an outboard. Yes, the idea behind a fin is to allow a greater X dimension (higher mounting) however a Dole fin isn't quite the right design to take advantage of the idea. Anytime a fin runs in or even near the surface it will create drag.

The fin isonly a tool. The fin, in a correct design, allows you to change the set up to take advantage of the prop technology.

Re: question about doel fins

where is cavitation plate in relation to bottom of transom? what type and hp motor? size boat? current wot rpms?type and size of prop? useful info for useful answer. imho foils suck.

Re: question about doel fins

The boat is a 16ft flat bottom jon boat with a 30hp 2 stroke tohatsu. Top speed without fins is 22mph with two people and full dress. With fins 19 mph. Battery,6 gallons of fuel,fishing gear etc..approx 1200lbs total weight boat,motor,gear,people,etc.. As for the prop the guy at the marina said it looked like a 10 or a 11 pitch. The prop may have been my whole problem it was in rough shape when I first got it. I straightened it out the best I could but knowing it was not right I have had it sent off to a prop shop and am expecting it back from the shop this thursday. They are supposed to rework it for me (the right way,lol) and increase the pitch by 1. So I guess it should come back an 11 or 12 pitch. There are no indicators on the prop as to what the pitch is only three letters. Dont remember what they were. The engine is an 82 model without the thru exhaust hub. Just a nut and a cotter pin. Will try without the fins when I get my prop back from the shop this week. Would be great if that was my only problem. Cavitation plate is about 1 or two inches below the bottom of the transom. As for wot rpm I currently have no tach. Been on the look for one but really cant afford a new one right now.

Re: question about doel fins

The AV palte should be no lower than even with the keel. If your motor is indeed that low then putting on any fin will create problems. With the motor that low you should have water spraying up the transom.

Re: question about doel fins

move the motor up untill you start losing performance. less drag equals better performance

Re: question about doel fins

I had the cav plate even with the bottom and it was worse than what it is now. The mechanic at the marina I go to told me on a flat bottom boat the cav plate should be 4 inches below the transom. I moved it two inches below trying to squeeze everything I could out of it.

Re: question about doel fins

I guess I will know more when I get my prop back from the shop tomorrow evening. Getting this setup just right has been aggravating. Did the marine tech misinform me or does 4in below the transom for a flat bottom boat sound right. Like I said my cav plate is about two inches below the bottom. He also told me the cav plate should run about 4in below the water line at full plane. I myself do not know. Its just confusing to get mismatched information and trying to figure out which way to go. All that I know is when I lowered the engine it helped and did not slip nowhere near as bad as when it was even with the bottom of the boat.

Re: question about doel fins

Plate even with the bottom is considered a starting point.Some motors end up an inch or more higher.A good prop should get you there.Your repaired prop should do better.If you need more the Turning point Rascal is an excellent prop.Stainless performance at Aluminum prices.
Maybe you didn't understand what mechanic said I don't think there any motors you could get the plate to 4 inches below . unless you put a long shaft on a short transom boat.

Re: question about doel fins

You are very right steele, Matt will you send us a picture from the stern even with the bottom of the boat, that would make it easy.

H

Re: question about doel fins

Will try to take that pic for you tomorrow. Got the prop back today,took off doel fins and it ran 24mph. Was by myself though. Usually have atleast two people on the boat so I guess the real test will have to wait for the weekend. I could raise the tilt to the setting before the last setting. A little higher than before so the prop did help somewhat. It also had a much much better take off. I just wish I had the prop fixed before I lowered the engine because raising it back up will be more difficult than lowering it due to the fact that I have no jackplate.

Re: question about doel fins

IF you rest the bottom of thefoot of the motor on the ground you can crank the bow up with the front roller and it makes it much easier.

H