prop slip check please

[ScottinAZ]

I would like to get a reality check here that the prop that I have is the right one for the boat, I think it is, but would prefer to know that I am not running with excessive slip, or just wasting gas.

Boat is a 1982 Imperial deep v (V183) with a 1982 Evinrude 115 outboard on it. I was running it today, and got up to 36.5mph according to GPS. (pitot speeds correlated nicely)

Specs on the boat are as follows

WOT MAX 5500RPM
1150 empty weight
~150 lbs of misc gear (anchor, PFD's, Battery, misc "junk in the trunk")
20 gal gas (give or take a gal or so)
300 lb Me
prop is a Solas 13.25x17


At 4500 rpm I was running at 30mph, give or take. all other specs are the same

I do not know the specs on the engine gear ratio, nor do I know the deadrise angle on the hull (I dont even know how to check the deadrise, but I could check it if explained how)

 

[muchco]

Re: prop slip check please

Hi, the gear ration on your 115 if stock would 2:1 this being the case your slip is as follows: 5500rpm/2*17*60/5280/12= 44.27mph then take your actual speed of 36.5gps/44.27=.825 or 17.5% slip which is a little on the high side IMHO. I would like to see in more in the 10% - 14% range and the lower the more effecient your set up is. There are other more knowledgable members on the forum that may be able to give you more insight and their opinions as well.

 

[ScottinAZ]

Re: prop slip check please

I have no reason to believe that the gears are other than stock. Thanks. Im sure hwsii will be along sometime, and run it through his sheet

I also ran the numbers for 4500 rpm, and am coming up with 17.5% slip there as well, so at least its consistent

I was also thinking it may be a bit low for top speed, but hey, it is what it is. I did seem to be ventilating a bit in a high speed turn though, so the motor may be a bit high (ill post cav plate measurements tonight), but alas, there is no more space to lower it, and Im not in a position to repower any time soon....

 

[muchco]

Re: prop slip check please

If it's ventillating now all the more reason to run the 4 blade SS. I'm sure that you will get ride of the ventillating with the 4 blade prop and if it runs more efficent at higher rpm with the lower pitch you most likely will not lose much speed if any at all.

 

[junior1113]

Re: prop slip check please

bad set up could result in excessive slip. thats where id start. where is cavitation plate in relation to bottom of transom? can u post a pic of this? if too much of the motor is in the water this creates drag which makes slip higher and rpms lower. if that were my 115 it would run at 5600-5800 with todays ethanol problems. happy boating

 

[muchco]

Re: prop slip check please

Just food for thought. Hypothetically if you were to put on a 14 pitch 4 blade SS prop and could turn it at 5800 with 12% slip this would be your approximate speed.
5800/2*15*60/5280/12= 41.2mph *.88 = 36.2 mph. I show the pitch in the calculation as 15 because you can add one inch of pitch to a cupped SS prop. This is all theroy but it gives you an idea of what you could see. There would be no comparison in regards to hole shot, handling, and a lack of cavitation. Good Luck, I'm sure one of the other guys will jump in.

 

[ScottinAZ]

Re: prop slip check please

ok, heres a couple of pics of the engine / hull relationship. It appears that the anti-ventilation plate is just even with the bottom of the keel. This may be a tad low, but not by a whole lot (from what I have read here).

 

[hwsiii]

Re: prop slip check please

Scott, you and Muchco are right your prop slip is right at 17% at 4,500 RPM and 30 MPH. But I didn't notice any RPM for 36.5 MPH and as a normal course of events as the prop gets closer to WOT the prop slip goes down and I would expect your WOT RPM to be close to 5,300 RPM. It is very hard to tell from the pictures how high your motor is, but if you are experiencing ventilation in turns, is that with the drive trimmed up, theoretically you can get rid of that with cup on the trailing edge of the prop,which as Muchco says usually adds one inch of theoretical pitch to the prop, and thus a reduction in RPM and a slight speed increase. I can tell though that you cannot move the motor down anymore without a great deal of trouble.
Do you carry any heavy loads at times or do you intend on pulling skiers or tubes, and how is your hole shot and acceleration right now, is it very good or is it lacking. If you don't have a good hole shot or think you might need extra thrust you have two choices a Turning Point Hustler 3 blade or a 4 blade. The Hustler is a very aggressive aluminum prop with cup, but it tends to raise the bow and not the stern as most aluminum 4 blades will do. The 4 blade would reduce your prop slip as it has a lot more blade surface area and give you more low end acceleration all the way through WOT. You would need to drop your pitch if you picked either one of these. It just depends on your needs and uses.


H

 

[ScottinAZ]

Re: prop slip check please

I dont really carry high loads (except for myself) but I do intend to pull a tuber from time to time. I must admit to estimating on the rpm's though, as the tach crapped out on me (I will be replacing it soon) Until the boat gets on plane the bow does tend to rise quite high, so I may need to play with my trim settings (I also need a new trim gauge sender, as full up to full down indication is about 1/2 second on the switch, with a LOT of movement left) Yesterday was more of a shakedown than anything else, so thanks for the reccomendations, and I will most likely be getting a slightly more agressive prop here soon. Other than the Hustler, are there any others out there that may improve efficiency, but not cause bow raise?

 

[ScottinAZ]

Re: prop slip check please

ok, I checked Turning Points website, and for a Hustler for my boat, they are reccomending a 21 or a 23 pitch blade. This seems WAY high, as I am in the RPM window with the 17 pitch Solas that I have on there now. What does the board think on this. I would be more inclined to go DOWN in pitch from what I have now, not UP in pitch.

 

[hwsiii]

Re: prop slip check please

Scott, that is way more pitch than your boat and motor can handle, you are right. But I believe it is prudent at this time to wait and let's get some real RPM and speed readings. Without knowing these numbers we cannot accurately predict which prop and pitch will be best for your boat. And I can tell from your latest posts that you probably don't need a Hustler, but a 4 blade would fit your needs better because of the bow rise. BUT, are you tucking the lower unit under the boat for takeoff, as that is the way it works best in most cases , and as the boat starts to transition to plane you start trimming the motor up.


H

 

[ScottinAZ]

Re: prop slip check please

im fairly new to boating in general, and power tilt and trim specifically (last boat was manual trim, so what you see was what you got. I trimmed the motor down and pretty much left it there, so there may be some speed still left in this combo. next time out, ill play with it a bit more and see if I can get it with less slip. Judging by where my trim gauge works, I was pretty high on trim. Next trip Ill have the wife check the motor and tell me when it stops trimming down, and then we will adjust it from there. Thanks all

 

[hwsiii]

Re: prop slip check please

Scott, at high speeds you want to trim it up until it starts to ventilate then trim it down just enough to where the prop isn't slipping anymore for max speed.


H

 

[ScottinAZ]

Re: prop slip check please

Roger that, Ill definitely give it a shot next time out. I assume I can tell its ventilating when I hear the engine start to rev a bit higher, at the same power setting on the helm

 

[hwsiii]

Re: prop slip check please

That's it. LOL


h