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I have an 83 Wellcraft Offshore 248 with a 260 mercruiser I/O. At 2500 rpm it only gets 9 knots (not on plane yet). When I get to 3000 rpm and on plane it gets 22 knots. I can't seem to keep it on plane, if I drop the throttle even a little it drops off plane and back to 9 knots. My question is will tabs help me stay on plane at lower speeds. I'm getting beat to death on the way out or it takes 3 hours to reach blue water.
for sure and on this boat double since it is rear end heavy with a big auto v8 and outdrive. I have a set for sale in Texas. that i never used on a project.
They should really help you..You have a lot of choices in brands and types. I liked the Lenco electrics I installed about 6 to 8 months ago on my 22 footer. One advantage to the electrics is there is nothing mounted in the bilge to rust. Good Luck..
They should really help you..You have a lot of choices in brands and types. I liked the Lenco electrics I installed about 6 to 8 months ago on my 22 footer. One advantage to the electrics is there is nothing mounted in the bilge to rust. Good Luck..
The real solution to your problem does not lie in tabs. Tabs will help a bit, not all that much, but they are not going to deal with the real problem. You're giving a guy with a broken leg a cane. He will walk better but it won't fix his leg. You need to set the bone and put on a cast. What you need to be looking at is your prop. Come down in pitch by at least one inch and possibly two and your problems will be solved. If you now have a 3-blade and go to a 4-blade it will be even better.
Thom
"I command thee, O fish of the ocean, rise to my bait"
While I agree with Thom, I would also get the trim tabs. By getting the bow down and more of the hull in the water, your going to get a smoother ride in the chop. It also reduces porpoising a lot with stern heavy I/O boats.
Almost weekly I run my 21WA through the Cape Cod Canal (7 miles long) which has a 10 mph speed limit, a -5 to +5 knot current, and some pretty rough water. Without tabs, when I'm going against the current, I can do 7 or 8 knots (too slow) before getting on plane or get on plane at 17 mph (speeding, at least it is when the canal patrol is around). With Tabs, I can get better control the speed at which I get on plane. And, as I wrote, the ride with the bow down is much better.
A few weeks ago, my tabs stopped working because a coupling came undone and the fluid ended up in the bilge. My wife refused to get back in the boat (to go back through the canal) and caught a ride home from the beach. Lesson: If you don't want your wife on the boat, don't use your tabs.
Definitely check into a lower pitch prop to go with the tabs. I run a 23' flybridge cruiser with an I/O. I put tabs and a 17 pitch prop on it years ago - made a world of difference in handling, performance and economy.
Thom is absolutely right. The prop is the place to start. My dad had the same boat and tabs would be a good idea, but dial in the prop first. Also, what is the condidtion of your hull, is it fouled?
This can make a big difference.
I don't have a problem getting on plane, if thats what the prop change suggestions are about. What I would like, is to be able stay on plane below 20 knots. Maybe I am missing the point about keeping the bow down to cleave the swells. What happens is when I am running into the swells, I keep dropping (read that SLAMMING) off of the swels that are rolling out from under me. When I am in the calm bay waters, 4000 rpm will get me about 34 knots, way faster than I would ever care to go.
It is the prop but the reason for telling you that is a bit hard to explain. I'll try.
What you have is a gap in the speed your boat is able to go. You've told us that at 2,500 RPM you are making 9 knots and that at 3,000 RPM its chugging along at 22 knots. I'd be willing to bet you can't give me an RPM at which you will be moving 12 knots, or one for 15 knots, or 17 or 19 for that matter. There is a whole bracket of speeds you are not able to run and hold. The reason for that is really pretty simple but not simple to grasp.
Props tend to be very efficient at very high speed, and pretty efficient at the mid range of prop speed, but not so great in the lower speeds. Basically the slower you are going the more they slip You hull has a strange looking curve in its power demands too. To move at very low speeds requires very little power, the power required to move at progressively higher spees increases dramatically until the boat rolls over onto plane, at which time the power requirement drops way down and then the power required to move at faster speeds is a lot closer to a moderately sloped straight line until you hit your maximum speed. The third thing you have to keep in your mind is the way your engine puts out power. Very little power is produced at low RPM, the power increases at first gradually as you go up the RPM scale, then it starts making big power in a hurry as you approach your upper RPM limit and then it falls off rapidly. OK, now keep those three things in mind, propeller efficiency, hull power requirement, and engine power output. Think of them in relation to speed and keep in mind that speed is always measured as a function of time, and that if power requirements are in equalibrium with power output you will hold a sustained speed. If power requirement is lower than power output which can be transfered to the water you'll accellerate and if power requirement is greater than power which is transferable to the water you will slow down. Got it all? Cause now I'll tell you what is going on and how a prop change fixes it.
OK, first at very low speeds the hull is easy to move through the water. The prop is slipping quite a bit an as a consequence the prop shaft speed (RPM) is higher than it would be if there was a straight hookup. The relatively fast prop shaft speed means that the engine is allowed to operate at a bit higher RPM that it otherwise would and that means that it is up there a little higher into its power curve. At these very low speeds you have adequate power to sustain your speed and all is well. Now you beging to move up in speed. The hull is requiring more and more power to move it forward (overcome drag of the whole bottom being wetted) amd the prop is slipping less which means that there is a greater demand on the power output from the engine. The engine is operating low in its RPM band but its still not got to the point where power demand is greater than available power. This situation remains in effect for your boat/prop/engine combination up until you hit 9 knots at 2,500 RPM. Up until that speed you have power demand, created by hull drag and expressed to the engine as a result of prop efficiency, that is equal to or less than potential power output from the engine. The engine is not only able to make the power needed to sustain speeds up to 9 knots but in fact it is able to put out power in excess of those needs so you have the ability to accelerate. All is well with the world.
Now you want to go 10 knots, or make that 12 knots if you like, or any other speed up to 22 knots for that matter. At that speed (I'll just use 10 knots) the drag on the hull is just a bit more than it was at 9 knots and the prop is just a bit more efficient because its spinning just a bit faster, and in fact the engine is also putting out just a bit more power because its spinning faster. The problem is that the engine is not spinning quite fast enough to put out the power required, its close but not quite there. Remember that hull drag goes up fast, prop efficiency goes up in pretty much a straight line, and that engine power output is flat at first and then comes on stronger and stronger as RPM goes up. You're still in the flat part of the engine power output curve so the power simply is not there to hold the boat at 10 knots. The power still isn't there at what would be the RPM necessary to go 13 knots either, nor is it there for 15 knots. The next time your combination will be in equalibrium will be when the engine speed has got up to 3,000 RPM and a hull speed of 22 knots (the reason you are able to pass though the dead zone between 9 and 22 knots is prop slip in case you were wondering - basically the engine is artificialy allowed to rev up and the prop just sort of catches up as speed increases).
So, what can you do to address the large dead zone? The ansewer is simplicity in itself. First there are things that you can't do. You can not do anyting to decrease hull drag, and that includes putting on trim tabs. Trim tabs, which are the next best thing to sliced bread and something that you shurely must have, never decrease drag. No matter what you do with tabs they increase drag. So, you can't do anything about drag, and at the same time you haven't done anything about low RPM engine power output (which would be very very expensive to address) so the only avenue opened to you is to simply get the engine spinning faster. After all, its not a matter of you engine not being able to make enough power. The engine can certainly do that, the problem is that the engine needs to be spinning faster to make the power you need. The light bulb just came on, didn't it. You can get the engine spinning faster by simply going to a prop of a lower pitch. That will allow the engine to run at a speed where it puts out the power necessary to meet the demands of the hull.. The reason to go to a 4-blade prop is that it is more efficient, in that it will allow less slip, and that will help you hold your top end speed as well as give better hookup in the mid range.
So that's it, get that prop pitch down just a bit and it will allow the engine to spool up to a point where its power output can match power demand. This will allow you to maintain any speed from dead stop to the fastest the boat is capable of. I don't even want to begin to tell you how long it took me to come to understand this but a great hint is available in this, did you ever notice that boats that have lots and lots of power have no problem going any speed they want? They have no dead spots. You could go that route and just replace your engine with one of greater power, or you could modify yours to put out more power, but both of those options require lots and lots of money. The alternative is much less expensive, just prop your boat so that the power your engine is capable of putting out is available at the hull speeds where you need it.
Now you understand?
Thom
"I command thee, O fish of the ocean, rise to my bait"
I don't think changing your prop size will matter that much. You'll see a modest improvement, but I don't think you will be happy. Bottom line - if you can develop enough RPM at WOT to be within manufacturer's specs, chances are you won't see much benefit from changing props.
The big speed swing from 2500 to 3000 rpm's has to do with running angle and s/l ratio - wetted surface is only a small factor.
Running angle - while you are trying to get on plane, bow pointed at the sky, your prop isn't perpendicular to the water's surface, so it is running less efficiently. That's why outboards and i/o's trim out once they are at speed.
Quick physics lesson - s/l ratio is the ratio of speed to the square root of your WATERLINE length. Trawlers and sailboats and other fiull displacement hulls care alot about this number. Once you're above an s/l ratio of 1.34, your reuire a lot more horsepower to drive a boat past it's hull speed. This is about 6 knots for your hull. Once you reach an s/l ratio of 2.0, the hull tries to climb it's bow wave. Planing hulls have the necessary shape and horsepower to "get over the hump". While trying to come up on plane, prop slippage is still high (see Thom's explanation) your can't develop max HP or torque (again Thom was right) and you are making your engine work really hard. Once you are over the hump, slippage goes down, required hp goes down, and gas consumption goes down.
Here's the part where I think tabs make the most difference and a new prop won't help. Underpropping your boat (to give it more power, essentially like using a lower gear) will make it easier to get over the hump when you slow down and come off plane, but it WON'T help much in allowing you to drive at lower speeds without sliding back off plane. While some props offer a little more lift than others (like increasing # of blades) none will match the lifting ability of tabs.
My boat slips off plane at about 19mph. I accelerate to 22 mph to get it on plane, and then slow down to 20 to cruise. Once it drops below 19, the boat starts sliding off plane and if I don't quickly accelerate, I have to start over.
With the tabs down, I can slow it down to 15 mph and still stay on plane. Keep in mind my boat is about the same size but heavier than yours (about 6,000#) and I only have 9x9 tabs.
About drag - When I am cruising at 20 and it's totally calm, I will retract the tabs. At this point the drag is negligible, since they are out of the water plane, and my speed will creep up to 22 and my gas consumption will go down.
Bottom line - tabs make a huge difference.
My gut feeling with a boat that size with one engine is the prop. I think with that one engine your minumum plane rpm should be closer to 3400 than 3000. I would try the prop. And of course the tabs will always help you get on plane quicker - I expect the tabs are probably more expensive and with the prop you can maybe work a deal somewhere to try it out. The prop though, will also help you increase engine life (less lugging") at the expense of some top end speed. I think you will end up doing both for the best performance of that rig and its worth the $ if you like the boat.
Trim Tab Question 
