only 3800 RPM at WOT - loss of power under load -- please HELP

[jrousell]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

I shoudl also say-- that even with a full load of people, gas and two kids on the tube I still will not get anywhere near the 5500 .. More like 20 MPH and 3800-4200 rpms...

lots of out lake time is spent pulling the kids on a tube...

if I went WOT to get higher rpms it'd be too fast for sure...

 

[reeldutch]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

get this one
http://www.americanparagliding.com/tinytach.htm

2C

good and acurate tach cheap and reliable

 

[reeldutch]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

if it turns out that with the tiny tach you still have 4900, and compresion is good then i would try a 15" prop.
i cannot believe you will want to go smaller than that.

 

[Dennisanoka]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

Jrousell,

Please keep in mind what ReelDutch mentioned about the ht. of the engine on your transom. You said your is "pretty" close in line with the bottom. Mine was about 3/4" below, max RPM trimmed was maybe 5200. I raised it to 3/4" above the bottom. WOT trimmed is now 5500-5600. So depending upon your definition of pretty close changing just 1.5" is alot. I have the 90HP version of your motor. I know this is not the prop forum but in my case the 13x17 is my best match 41mph (GPS) If I put on a 13x19 I get the same speed at 5400 but it cavitates there and I can go no higher. If I put on a 13x15 I get 5800.(could go higher RPM but I stop there) I get 37MPH but it comes out of the hole faster. All of these findings are based on the ht. of my engine if I lower it even 1" or less they all would drop.

I am sure my 16' Lund Renegade is lighter than your rig. So as you said try the 15 and if it doesn't get t you there consider raising your engine a hole or two.

 

[jrousell]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

I will try to attach a picture of the transom that I took this spring when I was changing lower uinit oil... -- not sure if this helps or not... the motor is trimmed up a bit( as you can tell)

 

[jrousell]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

I should also say that I do not think it should/could be much ( if any) higher than it already is. When I make semi sharp turns now even trimmed all the way down it tends to cavitate sometimes... Not a ton and I normally am not turning too sharp at WOT anyway, but I wouldn't want to increase that situation.

 

[Dennisanoka]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

On the engine bracket there are 6 holes/positions in which you can slide that bar in and out of. (I'm going to get laughed at because I don't know the name of that bar and such) In the picture, it's kind of fuzzy but do you have that bar in the hole closest to the transom now?

I rahter doubt I could ever turn very sharp at wot without cavitating. That also goes for racing boats, when they get to the turn they are trimmed down for that very reason.

 

[jrousell]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

attached is another image ( close-up) of those holes and which one is used now.

Forgive my stupidity- I thougth that I woudl have to change the bolts mounting to the transom in order to raise the plate vertically...

sounds liek it couldn;t hurt me getting a more exact measurement before I proceed with a different pitched prop.


and yes- I know most boats cavitate at tight turns, but mine does thios even with trimmed all teh way down, so I don;t necessarily wnat taht to start happennign on moderate turns too

 

[jrousell]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

woops - - forgot the attachment

 

[iwombat]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

Tough to tell from that pic, but it kind of looks like maybe it's too high on the transom. Need a better picture though.

I set mine one hole too high and dropped 200rpm at WOT. Back down and it's better, but I still need to reprop.

 

[darrklim2]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

If it cavitates in turns I would lower that pin one more hole and see if there is any difference. This may also give you a little more rpms. I know I lose about 100 rpm when I go out/up one more hole than what appears to be the optimum for my set up.

 

[Dennisanoka]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

Here's my 3 cents. I always have the bar set next to the transom, that's why I have a trim feature. I suggest that you set it in the first hole as well, then measure where the cavitation plate is in respect to the hull. Now I understand why you cavitate in turns, it's because you effectively always have it trimmed up. Trimming the engine and raising it on the transom have 2 totally different effects. Raising it up on the transom reduces drag, trimming it raises the bow. Too much of either produces unwanted results. I strongly believe that you firsts want to determine the proper engine ht. on the transon them trim for wot. So how would you know when it is too high? The most obvious one is if you have it trimmed ALL the way down and on take off or a moderate at speed turn you cavitate, that is too high.


Thus concludes my 3 cents

 

[reeldutch]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

before you start changing the setup verify your tach is showing treu rpm.

600 rpm is realy a lot off rpm to gain.

rule of tumb is for every ft setback you can raise the motor 1 inch and gain 200 rpm.
you might want to raise the motor to 1.5" with the anti ventilation plate above the keel.
thats about is and you might have gained 200 rpm max.
so now you are at 5100
still need 400 more means dropping 2 sizes and thats where the problem is.
smaller then 15 i never heard off on a 19' boat and 115hp evi

how is the condition of your hull?
if the boats hull has a warp or hook in it it will be effecting overall performance.

before you buy a new prop check all these things out.
-rpm treu?
-hight of motor antiventilation plate
- hull condition
- prop condition
- engine performance (link and sync)

 

[Dennisanoka]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

you're right ReelDutch I should have only shared 1 cent 3 was too much.

 

[reeldutch]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

do me a favour and mesure the hight of the transom.

looking at your pic i think iwombat might be right.

looks to high but hard to tell.

 

[Fl_Richard]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

My 2 cents..

I agree with Dennis. It looks like the trim is too high, especially for a light aluminum boat. Put it in the lowest hole and utilize the power trim for the best performance.

Verify Tach is working properly. (I think it is - see below)

just ran a prop slip calculation. (using 2:1 gear ratio)
17" pitch with 2.0 gears and 5800 RPM = 47 MPH with 10% prop slip = 42 MPH
17" pitch with 2.0 gears and 4800 RPM = 39MPH with 10%slip = 35 MPH --This sounds like you!

Provided you have 2:1 gears I think your either lugging or the engine still needs some work.

 

[Dennisanoka]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

Is it not true that when he measures the anti-cavitation plate ht. that it needs to be done with the engine in the full down position, ie, with the bar set nearest the transom? (not where he has it now)

 

[72SideWinderSS]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

I agree with Dennis and Fl, You really should try moving the thrust pin as close as you can to the transom.

With that much initial trim it is like trying to climb out of a hole all the time, I do not believe you are ever fully planing out as set. You even said with you in the back it would not plan.

The picture revealed you are mounted (just a WAG) one hole up. Is this the case?

Most accurate method of measuring where the anti-ventilation plate is at:
1) Place a level on the keel
2) Raise or lower the tongue jack until keel reads level.
3) Place level on anti-ventilation plate and trim until level.
4) Measure distance vertically between A?V plate and keel.

Proper use of trim:

1) Trim all the way down for take off ( you may find you do not need to but for testing I would)
2) Advance the throttle full.
3) As boat comes on plan, start trimming up until pull to right stops.
4) Adjust trim up or down to make for the best ride.

When testing for WOT trim up until the the speed no longer increases but the RPM's will. This is your WOT RPM.

 

[jrousell]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

Hull condition is great
Prop condition is excellent also

I will measure the difference from the plate and the bottom of the hull and post it asap

 

[jrousell]

Re: only 3800 RPM at WOT - loss of power under load -- please HELP

replying to FL_Richard

yes- I hit about 36-37 MPH when I get 4900 RPM on my tach right now


dropping the thrust pin down looks like it might be an easy thing to try -- is this a correct assumption? if it is easy maybe I shoudl go ahead and try taht next time I have it out of the water...

ANy tips for easiest way to do this?