Top End Speed Problem

A quick summary of my problem: Three years ago I purchased a 14' Crestliner Sportsman tiller fishing boat (rated for 35 HP) with a 15 HP Yamaha, due to lake restrictions. I was disappointed with the top end speed (16 mph), so the marina swapped me the 11" pitch (3 blade) standard duty prop, for a 9" pitch prop. This successfully kicked me up to where I wanted to go easily. My top end speed became 24 mph, with myself and another angler aboard + gear (5100 rpm). Great. However...it would take me about 45 sec. or more to get up on plane. Not good!

To solve this I installed a Doel Fin on the engine. That worked marvelously! Got me up on plane in <10 sec.. However, I lost 8 or 9 mph on my top end! And my top end rpm's increased to about 5300.

To try to solve this new problem, I got a "Mini-Jacker" jack plate. I completed installing this two days ago. Here's what the end result looks like:



And here's the relationship between my sonar's transducer and the port side Doel Fin. The transducer skims the top surface of the water, so I figured this relationship was pretty close to that, with the fin about 1" lower:



Launched yesterday and was totally disappointed once again. Lost another 2 mph on the top end @ 5080 rpms. Now all I'm getting is 15 mph (GPS + sonar paddle compares to withing 1 mph). It appears that the water is spraying just under the upper fin which is ABOVE the Doel Fin about 2". Not sure what they call that fin, but it's not the cavitation plate.

I'm planning on raising the engine up another 2" to more or less get the DF out of the water (at top end speed), without jeopardizing the water inlets just below the DF. Instead of unscrewing and rescrewing the jack plate, I'll just use wood blocks of varying sizes to see what happens. Does this sound like a logical process? Am I missing something?

Sorry for such a long post. But I'm hoping someone of you can steer me in a more correct path, to getting back my top end speed, yet maintaining the quicker hole shot. Thanks ahead of time.

Re: Top End Speed Problem

Get rid of the foil, bring the motor back down to where you got the good speed and go with Smart Tabs. Done deal. You'll be happy

Re: Top End Speed Problem

from the looks of it you have been running a long shaft motor on a short shaft transom. what is the measurement of the transom. from top to the center keel of the boat? also the measurement of the motor, from where it sits on transom to the av plate with the fin? this could be your problem all along.

QC and i were posting at same time, i'm in agreement, but would like measurements before committing.

Re: Top End Speed Problem

I agree too. I missed the shaft discrepancy and was responding only to the good top speed numbers before the foil and the plate . . . Odd to me that it would suffer to plane if it also got good top speed and it was too deep

Re: Top End Speed Problem

Definitely a long shaft vrs. short shaft issue on your boat.

Your "jack plate" cured that issue. The alignment looks right on now. No unnecessary drag.

Lose the fin and try it out.

Re: Top End Speed Problem

So, you folks do not think I should try to raise it any higher with the DF attached? Easy to remove it, but what about the 4 holes in the cavitation plate? Don't I have to fill them first?

Re: Top End Speed Problem

Crestliner2007 said:

So, you folks do not think I should try to raise it any higher with the DF attached? Easy to remove it, but what about the 4 holes in the cavitation plate? Don't I have to fill them first?

Click to expand...

That part of the lower unit you attached the fin to should be level with the bottom of your transom. It looks like it is in your picture. You don't need to fill the holes.

Re: Top End Speed Problem

Just a follow-up on this problem. Tested the boat after removing the Doel Fin. Went from about 16 mph up to 20 mph. A significant improvement indeed! At WOT, the cavitation plate just above the prop is still submerged by about 2" I would guess. Not sure if raising it further is going to help much, but I figured I'd slide in a 1 1/2" block under the engine bracket and see how that works.

What concerned me is that the rpm's jumped from about 5080 up to 5400 after removing the DF. The max rpm for this engine is 5000 +-50. Kind of scared me, but I didn't notice anything unusual with the engine sound or performance.

One thing did happen that bothered me most: When I turned into a light wind (about 10 mph) at WOT, the rpm's dropped to 4840 and the engine did sound like it was loosing power. Did I do something wrong???

Re: Top End Speed Problem

Cool!! We really need to know where the Anti-Ventilation plate (cavitation plate) is relative to the bottom, not how you perceive its height relative to the water . . . Too subjective . . .

RPM must go up and down with speed change. When it goes down that indicates more load, so the engine does sound like it is working harder . . .

Re: Top End Speed Problem

our guys here seem to have you on the right track......

(yes the anti cav plate must be at the bottom if the keel height....one or two inches is a lot)
but im confused when you dropped the pitch of the prop....that should have givin you a hole shot........and more rpm's (possibly more top end if you were really over porpped......

did i read that backwards?.....qc? td? silver tip?

Re: Top End Speed Problem

If you don't need the DF then pitch it, no reason to have it on there if it's not helping. DFs are designed to be out of the water when set up correctly, if it's in the water at speed then it can create a great deal of drag and other issues. Keep raising the motor until the A/V plate is at least at the surface when on plane, if not a little higher.

Re: Top End Speed Problem

QC said:

Cool!! We really need to know where the Anti-Ventilation plate (cavitation plate) is relative to the bottom, not how you perceive its height relative to the water . . . Too subjective . . .

RPM must go up and down with speed change. When it goes down that indicates more load, so the engine does sound like it is working harder . . .

Click to expand...

Not quite sure how to answer this; being a deep-V, the stern has a shallow V to it (as you can see in the photos), with the keep being about 1" below the lowest part of the V. The Cavitation plate of the engine is just about even with the keel. This is below the skimmer sonar transducer you see in the photo. I was under the impression that the cavitation plate was suppose to ride on the surface of the water? It is definitely below the surface at WOT the way it's positioned right now.

Also, without the DF on the engine, I'm back to almost a full minute before she planes out.

As far as the prop pitch goes, what came with the boat was the "regular" duty, 11" pitch prop. The marina swapped that one for me, with the 9" "heavy duty" prop I have on there now.

Some folks around my area have told me that I need to go to a "bigger" prop. I'm assuming they are referring to a larger diameter prop. In looking at my engine, I don't see as how this would be possible. Not enough clearance.

THANKS ALL, for your continued support!

Re: Top End Speed Problem

oops! said:

our guys here seem to have you on the right track......

(yes the anti cav plate must be at the bottom if the keel height....one or two inches is a lot)
but im confused when you dropped the pitch of the prop....that should have givin you a hole shot........and more rpm's (possibly more top end if you were really over porpped......

did i read that backwards?.....qc? td? silver tip?

Click to expand...

When I dropped the pitch, all it did was give me greater top end speed. It did nothing to improve my hole shot. Which is why I went the DF in the first place. And don't forget, this long shaft was bolted on the boats transom (which is quite low - see pics), which put the prop way below the bottom of the boat.

Re: Top End Speed Problem

oops! said:

our guys here seem to have you on the right track......

(yes the anti cav plate must be at the bottom if the keel height....one or two inches is a lot)
but im confused when you dropped the pitch of the prop....that should have givin you a hole shot........and more rpm's (possibly more top end if you were really over porpped......

did i read that backwards?.....qc? td? silver tip?

Click to expand...

Sorry....I don't know what qc, td or silver tip mean?

Re: Top End Speed Problem

I'd try to put the cav plate one inch or two over the bottom of the keel. You have setback now so it can run a little higher than even.

As for higher rpm's put your old prop back on. If you do have a long shaft on a short transom your adjustments (prop and fin) where off from the get go. Start back near the beginning.

Plane time? Is your motor trimmed out trying to get high speed? It looks like it is in the pics. Drop it down two holes and see where your at.

I think you'll get real close with what you already own but I also believe a set of smart tabs will make a big improvement if you want to invest some more.

Re: Top End Speed Problem

NSBCraig said:

I'd try to put the cav plate one inch or two over the bottom of the keel. You have setback now so it can run a little higher than even.

Click to expand...

This seems prudent . . .

NSBCraig said:

As for higher rpm's put your old prop back on.

Click to expand...

I am guessing this will kill your hole shot, but may depend on trim angle as noted below.

NSBCraig said:

If you do have a long shaft on a short transom your adjustments (prop and fin) where off from the get go. Start back near the beginning.

Click to expand...

I thought we were getting there with the height discussions . . . not sure why he needs to start over

NSBCraig said:

Plane time? Is your motor trimmed out trying to get high speed? It looks like it is in the pics. Drop it down two holes and see where your at.

Click to expand...

I think you mean trim it down two holes . . . one at a time may be a better plan . . .

NSBCraig said:

I think you'll get real close with what you already own but I also believe a set of smart tabs will make a big improvement if you want to invest some more.

Click to expand...

100% agree . . .

Re: Top End Speed Problem

Every step I take I try changing the trim adjustment and checking the top end speed (GPS + sonar paddle) and rpms (I'm using a "Tiny Tach" tachometer). In every case thus far, the second hole from the rear is where I get the best performance. This is where it is in the photos. This includes raising, blocking, putting the engine on the jack plate and removing the Doel Fin. So I'm pretty confident about the engine relationship to the transom. In any case, I'll continue to try trim changes whenever I make any future adjustments to the set-up of this rig.

As far as going back to an 11" pitch prop, this doesn't make much sense to me? Especially since I'd now have to BUY one (the marina switched props for me in the first year for free). I don't think I'd care to drop another $90. on a "maybe" solution. If someone could assure me that that would help, I most certainly would....but I can get 2 truck fill-ups plus 1 boat fill-up of gas with that money, so please understand my reluctance.

Re: Top End Speed Problem

The only thing I will add is that you should carry two props anyway . . .

Re: Top End Speed Problem

If it's taking that long to get on plane, with the bow high, maybe it's time to experiment with weight distribution. How's the boat set up now? Fuel tank? Battery? Tackle? Other junk?

On my 12' boat, that was critical. Since I'm running a 6hp on a boat rated for 10hp, weight distribution ended up being the thing that got me on plane and scooting along at about 15 mph, as fast as I could expect. I moved the fuel tank to the side of the middle seat, and the trolling motor battery up by the front seat on a little platform. Suddenly, the boat performed for the first time.

If I add a passenger, I have to trim out one hole, and only get about 12 mph.

Getting maximum performance when you're underpowered is tough, and small changes in how the weight is sitting can make big differences.

Re: Top End Speed Problem

My Sea Nymph is exactly the same CAT . . . with 5 bhp even more critical . . .