F150 Lost RPM's---Need Help

[riv]

I have a Crestliner 1850 sportfish with a Yamaha F150. Last fall I had a 21” prop on the motor and I was able to get 5500-5600 rmp’s and around 50 mph on the gps. This year with the 21” prop I am only able to get 4800-5000 rmp’s. I took it into the dealer and they can not find anything wrong and the motor and said it is with in spec. I agree it is with in spec after changing to a 19” prop, but last year it performed way better. They did change the prop for me at no charge to a 19” pitch. I am now able to get 5300 rpm with two guys and 43 to 44 mph. What could be wrong? Is there a throttle linkage adjustment or something else that could be out of adjustment? The only change I made from last year was a bow mount trolling motor, 2 batteries, and a charger. I did take the batteries and trolling motor off and tried it this year with an increase in rmp’s of 200.

What kind of speed are people getting with this boat and 135 to 175hp motors? Maybe this is all I should expect with the trolling motor and batteries. It just seams like there is such a change for last year. Please help!

[vikes40for60]

I too have been a little bit dissapointed with the RPMs on my F150. I run a similiar style boat, an Alumacraft TS175. Last fall during break in I had a 19 pitch aluminum and I was getting 5600-5700 RPMs and 48-48.5 MPH. Not bad performance, but not the best. This year I moved to a 19 Pitch Tempest and raised the motoe 2 holes to the third hole. This move seems to be the most dissapointing. I can only get 5500 RPMS (maybe 5600 if alone) and getting 45-46 consistently. The hole shot, bow lift, and wave control are great but top end and MAX RPMs are dissapointing. I cannot get the boat to porpose and max trim is easily reached. I am wondeirng if moving the motor up one more hole to the top position would make a difference? Any advice for my situation?

[REW]

It sounds like you need a prop with more bow lift and or you need to install transom wedges on your boat, to tilt the motor further out.
When you are at maximum trim, are you getting any cavitation?
If not, the addition of transom wedges might be just the thing.

Remember, some of these motors are a bit heavier, and can really use a bow lifting prop.

Take care
REW

[vikes40for60]

REW-

I assume you are talking to me? The answer is NO. I am getting NO cavitation at full trim? Wouldnt be better off raising the motor one more hole first? Or would dropping to a 17" Pitch also be an option?

[mdsranger]

riv, you could try aseafoam shock treatment. its 3oz. of seafoam per gal of gas. this will clean out any varnish residue from winter storage, also any carbon deposits. i would run at least 1/2 tank of gas thru like this. two buddies did this with excellant results, it may do the trick and won't hurt nothing. good luck, mike

[riv]

I tried several different stainless steel prop with very little difference in performance. I may try the seafoam, but I will have to wait until my tank is close to empty. I am still concerned with the huge loss in performance. It just seem like there is something wrong with the hull or motor. Thanks for the help.

[JamesM]

You said initially that you were getting better perfomance in the fall - was the temp significantly cooler when you were running in the fall? That could account for some of the drop-off in performance (the cool air/water will generally have a positive effect on your engine's performance - it's typical to be able to turn more r's in cooler weather than in the heat of summer).

James

[Classic2]

I have the same motor on a Lund 1700 Fisherman. I have a 19 pitch aluminum prop and have 6000 rpm's and get 50-51 mph (GPS). It is new this spring.

I have also experienced better performance in the spring and fall when the temps are cooler.

Chester

[REW]

I have to agree with the previous posts.
You absolutely lose performance, when the water warms, as well as when the weather warms, and as the humidity increases.

Cold water is denser. i.e. it is literally harder, and thus your hull runs higher in the water. Less hull in the water, the less drag, and the faster you run, and the more rpms you make on your engine.

The colder the air, up to a point, the more oxygen can be carried in the air. More oxygen makes more power.

The dryer the air, i.e. less humidity, the more oxygen can be carried in the air, and thus more power.

Tie all of the theee items together, and there can be a very marked difference in boat/motor performance on a 40 degree fall day, with the water temperature at 40 degrees compared to a spring day, with the water temps in the high 60s to low 70s and the air temps in the 70s with the humidity in the 50% range. Very large power loss due to these factors.

Generally, on my boat - there is about a 4 mph difference between the last run of the fall before freeze up - compared to the same run on the 4th of July in 88 deg temps with 88% humidity.

Then, when fall comes with cold water and cool weather, the speed and rpms come back up.

Take care
REW


p.s.
If you wonder about the performance of your motor; take the motor to your dealer and have them put a speed wheel on your motor.
Every motor is assigned a specific speed wheel. Typically a good dealer will have the correct speed wheel for the motor that he sells.
Then, all that the mechanic has to do, is remove your prop, install the speed wheel, start the engine, and run the throttle to maximum.
The speed wheels are calibrated to absorb so much power. In your case, your motor with a specific speed wheel, will turn this wheel at a specific rpm, if it is making the manufacturers rated power.
If it turns faster; it is making more power than the manufacturers rating. If it turns slower - it is making less power.

Now, if when the mechanic puts the speed wheel on the motor, and if it turns at the correct rpm; indicating that your engine is putting out rated power and you are still unhappy with the perforance, with your propellor - you should look at a different prop. i.e. assuming that you didn't hit something with your boat, or change the setup of the boat, or add more things to make the boat heavier.

Often, a very light prop strike can have some very serious effects on the performance of your boat/motor.
If in doubt - try a new prop of the same size, and manufacturer.
See if it is any different with a brand new prop of the identical size and type.

Take care
REW

[riv]

This spring the water and air temps were much colder than last fall. I am pretty sure that it is not the water and air temps.

[REW]

I agree on the cold water and cold air.
As I stated before, take your rig in and have it tested with the speed wheel or dyno. Then you will know exactly how much hp your engine is producing.

If the engine is producing the expected 150 hp, you have a
rigging problem, a prop problem, or a hull problem.

Eliminate them one at a time; and you will find your problem.

Take care
REW

[Unlogged T-Mac]

19 is the correct prop for that rig.
With a brand new boat, nothing in it and very little fuel a 21 might have worked ok.
We see that alot. But, when the guy puts all his stuff and a bunch of fuel in, another passenger or two, etc., etc.....it needs to be propped correctly.
19 is correct.

[riv]

T-Mac what speed and rmp's should I expect out of this rig. I just bugs me that I lost so much speed and rmps since last year.

[vikes40for60]

I agree and feel that 19 pitch is the correct prop for my boat. (5500 RPMS and 47 MPH in the third hole up) Will a move to the top hole give me 200 more RPMS? If yes what are the downsides of moving the motor that high?

[Good stuff]

Rew/T-Mac
Please share with us what could happen if you have a prop strike I did last year had the skag repaired and replaced the prop (alum) with an after market. I now have a boat that I can not run WOT without exceeding RPM 5200 (MAX) I have also lost 6-9 MPH. The new prop is 13.25 x 17 pitch, same size as factory unit not sure about rake and cup, I'm running a 2001 125 Merc on a 2002 1800 Fisherman.

Please also share your recomendations regarding what hole 1st, 2nd, 3rd that you would mount the motor in, at this time my motor is mounted in the first hole (Top).

Thank you for your help this is a great place.

[Unlogged T-Mac]

It depends on your load , but Yamaha shows 5800rpm and about 48 mph with their test load on an 1800 Fisherman/F150. They had 20 gals of fuel, a bow mount w/ 2 troll batteries, on-board charger, T-8 Yamaha kicker, gear bag and @ 1000 ft of elevation.
They used the Yamaha 13 3/4 X 19p black SS prop.

[Me agin]

of course....the Lund will be faster ...ya know... LOL.
;)

[T Mac Mt]

Question, "Good stuff"...is this speed loss shown via GPS?

[Goog Stuff]

Yes, GPS and gage

Thanks