Loss of "Grip" in turns

[geeco1]

I have recently purchased a 17 ft Blue Fin Sportsman with an 1988 85hp Force. As I start to feel this boat out, I'm sure that I'll have several questions, but here is the first one.

When I am running at WOT I can turn to the right with no problem, but when I turn to the left the prop loses "grip" in the water. I can hear the engine rev-up but the boat will not turn as tight and slows down. When I staighten back-up, the prop "digs in" again and off I go. Any ideas on how to correct this?

Thanks,
Geeco

 

[crobar]

Re: Loss of "Grip" in turns

One thing you can check is the down trim on your motor. If your trim control is manual then you might try moving the trim stop rod to a lower set of holes.

 

[geeco1]

Re: Loss of "Grip" in turns

Ahhh.... let me add some more detail. It is power trim and i have it all the way down. I believe that it is a 21 pitch prop. If I trim the motor up very much at all it also loses grip. With a boat that I had several years ago, I could trim it up quite-a-bit and the bow would raise. On this boat, I can't get the bow to raise by trimming it up, it just loses grip. (sorry, I don't know what the term is).

Geeco

 

[hwsiii]

Re: Loss of "Grip" in turns

Geeco, it sounds like whoever owned the boat before you may have moved the motor up for maximum performance, and then when they sold it to you they may have given you a prop not capable of running as high as the other prop did. Is it a stainless or aluminum prop on it now.
A couple of pictures would help to see how high the motor is on the transom.

H

 

[geeco1]

Re: Loss of "Grip" in turns

hwsiii, I'll try to get some pictures this w/e. I know that the previous owner had the transom rebuilt, but I don't think that he was "savvy" enough to worry about performance, but who knows. It has a 3 blade aluminum on it now, brand new actually. It appears to be a 21 pitch.

BTW... I noticed on another post that you have a method for calculating the best prop choice. Is there somewhere that has the information that you are asking for? Such as Deadrise, gear ratio, RPM at WOT, etc. My motor is a 1988 85hp Force.

Thanks,
Geeco

 

[Frank Acampora]

Re: Loss of "Grip" in turns

Force engines actually have a short, or relatively short midleg.

You are experiencing ventilation in the turns and when you trim out/up because the anti-ventilation plate is too close to the surface of the water.

If your hull has a flat riding section in the center of the vee: "riding pad" and if the former owner raised the engine on the transom to where it (the plate) is one inch above this pad, then she will ventilate and cavitate. --A lot in turns, and you will not be able to use much up trim on the straights either.

You say the transom was rebuilt. Is it possible that during the rebuild, the transom was made too high? A difference of only 1/2 inch will cause problems on some hulls.

At any rate Cupping the prop will help some and if you can find one, the plastic black tapered "Flare" washer may help a little too. But these steps will only reduce the problem some. Ultimately, the only sure cure is to bring the cavitation plate down to the bottom of the vee or the bottom of the riding pad. Force engines are relatively intolerant of a high transom position.

HISTORY: In 1979, Chrysler developed the one piece lower unit you have on your 1988 Force. The midleg was one inch shorter than the midleg on the two piece lower unit it replaced. Customers were bitching like crazy because the engines were ventilating badly. Chrysler engineers quickly developed the "anti-Cavitation Flare Washer" to fit the back of the prop behind the tailcone and replace the tapered adapter first used between the prop and tailcone.

Since you do not have a dual exhaust lower unit, you can use the flare washer on your prop. BUT: As I said earlier, the only sure cure is to lower the engine on the transom so the anti-ventilation plate (cavitation plate) is even with the hull bottom.

 

[hwsiii]

Re: Loss of "Grip" in turns

Geeco, welcome to Iboats prop forum and we will try to help you as much as we can, but there is certain information that is needed in order to help you. Working over the internet and not being able to see the boat or prop the question is about makes it much harder to propose a solution to the correct prop that you need, and most people don?t know the make and model of the prop . When we get information in bits and pieces we continually have to look back and forth to try and piece the information together. The following form has all of the information that is needed to help you with your problems in finding the right prop. If you do not know all of the information that is asked for on the form please give us the most information you have available as I have a program I designed to find the right prop for any planing hull boat that needs this information. It requires time to find all this information when we have to look for it and in my defense I expect you to be willing to expend as much effort in finding the right prop as I do. We hope to be able to help you get the best prop we can for your particular uses, and this information is required to do that.
It is not as simple to pick the right prop as most people coming to this forum think it is, that is why we ask so many questions, it is for your benefit we ask all these questions. And if you have a stainless steel prop or are changing to one the blade geometry of the prop can make a lot of difference in speed and RPM. Most of my calculations are done assuming an aluminum 3 bladed prop will be used, when changing to a SS prop or changing within SS props we need to discuss what you are trying to accomplish.

Iboats Boat and Motor Info

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make and model of motor
7. HP and gear ratio of motor IMPORTANT
7A If you have a mercruiser and don?t know the ratio get your serial off the outdrive and go here http://www.sterndrives.com/mercruisersterndrives.html
8. Manufacturer?s recommended WOT range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS
14. Are you at sea level, or a much higher elevation
15. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.

H

 

[steelespike]

Re: Loss of "Grip" in turns

21" pitch sure seems like a lot.Do you have any rpm figures.With the right info hwsiii can help get the right prop but you shouldn't make any changes without a tach.If you read the stickies at the top of the page and get us as much info as possible we can get you in the ballpark until you can get hwsiii
his info.Terms like "about" "I think" etc should be avoided.Your prop numbers may appear on the barrel or under the prop nut.

 

[geeco1]

Re: Loss of "Grip" in turns

OK, Here is a picture of how my motor sits in relation to the bottom of the boat.

 

[Frank Acampora]

Re: Loss of "Grip" in turns

From the angle, it is difficult to tell the height of the ventilation plate. You need to run a straight piece of wood or whatever along the bottom and out to the ventilation plate to see the relation. If necessary, temporarily jack up the hull so it is off the back roller.

However, from what I CAN see, it sure looks like the previous owner did a botched-up job repairing the transom. A flat plate on the outside without anything on the inside is almost useless and he didn't do a neat job with the sealer either.

One other thing I notice is that the transom looks relatively vertical instead of angled back. This means that the engine which was designed for an average tilted transom is already tilted some toward out trim even while in full in trim position. If the ventilation plate is tilted up at the back while in fyll retracted position, then you are operating in a more-or less trimmed position at all times--not the way the engine was designed to work.