Help With Propeller Selection

[seawise]

Hi there

I have a 1989 28 FT bayliner with the 7.5L king cobra. The motor is rated at 4800 Max RPM and I am running a 15 1/2 X15pitch propeller and the boat @ WOT will do 4200 RPM and has a hard time planing and will fall on its face at 3900RPM. I was thinking about getting a 15 1/2 X 13 propeller? does anyone have any suggestions??????

thanks!

 

[hwsiii]

Re: Help With Propeller Selection

Seawise we need to know the horsepower and speed you are at WOT, as well as the gear ratio of your outdrive, as well as the model of your boat and the manufacturer and model of your propeller as well as if it is aluminum or stainless, then we can be of more assistance. Welcome to the forum.

H

 

[seawise]

Re: Help With Propeller Selection

The boat is a 2858 Bayliner 1989, it has 340HP the outdrive is a king cobra omc the gear ratio is 1.43:1 the propeller is an aluminum OMC prop model # 391535. As for the speed im at, its not very accurate the speedometer, but its reading around 28-30MPH

thanks

 

[hwsiii]

Re: Help With Propeller Selection

Seawise, I am showing an exceedingly high Slip Ratio (over 20%) for that prop, is the prop in good shape and is the bottom clean on your boat. With a slip that high some of the time it can mean you don't have enough blade surface area, but I question that highly in this case.
Are the motors tuned up and have you checked the compression in all the cylinders, because I am showing that you should be going a good bit faster than you are.
I really need a GPS speed to do any really accurate numbers on this boat, if I send you a prop test sheet can you take the boat out and write down all the speeds at the different RPM for me. The following is my guess at the full allup weight of your boat at this time.
Theoretically if you change to a 13" pitch you will gain your RPM up to right at 4,800, but there is something wrong with the numbers that I have from you, so I need a GPS reading. I show you should be able to turn a 15" prop at close to maximum RPM with the weight and motor that you have now.

H

 

[seawise]

Re: Help With Propeller Selection

Alright

sounds good, I sent you an email so if you could send me the sheet that would be great

 

[steelespike]

Re: Help With Propeller Selection

Generally if you want to get on plane and stay on plane you would go to a
4 blade prop.I'll assume you allready have trim tabs.Also assume you trim in as yuou slow.
I'm no expert but it would seem that a 13" prop would be extreme.
H has done his home work And while these programs can be fooled
by some of the unknowns.His is very comeplete.

 

[Tail_Gunner]

Re: Help With Propeller Selection

It makes you wonder that i will say. But as in Tony's thread his grady should turn a 15 by the numbers and thats with a fresh 350 mag. Here we have a older 7.5 and larger hull already 600 rpm under the rating and it fall's off plane quickly.

Remeber propping to max wot in reality increase's the boats lower and midrange response aka a 15" prop turning 4200 is going to place a far heavier load on the motor leg combo than a 13 turning 4800.

Id check the tach closely and if it is right...then your 13p is correct only tabs will help you from there. As to prop's there is little avaibilty 13 aka a large blade design..But i-boats does have a 13 stilletto and it would greatly improve things if your tach is right and motor is in tune.

 

[hwsiii]

Re: Help With Propeller Selection

Seawise, who is the manufacturer and model number of the prop you are running now, and is it stainless steel or aluminum. Sorry I forgot you already posted that. Can you also get a picture of the prop and possibly the bottom of the boat if it is not too hard. But please check the bottom of the boat either way for any major roughness, barnacles and things that should not be there. Thank you very much Steele, I appreciate your comment on my program. Tailgunner I agree with you on always running the motor at the Top of WOT RPM, it is less harmful to the motor and does increase performance from midrange and down. I am sorry about my past statements and agression.

H

 

[Tail_Gunner]

Re: Help With Propeller Selection

Dont worry about i am a bit sharp angled...and which i have had to say opp's sorry about that a few time's as now.. Dont over read this but when DH makes a comment pay attention...To bad Walleyehead is not around to much anymore...He would be more than capable to assist you with your table's and conclusions.

 

[hwsiii]

Re: Help With Propeller Selection

Tailgunner, I do pay attention to him, even when he says things I don't understand the reasoning behind.

H

 

[seawise]

Re: Help With Propeller Selection

Hey Hwsiii, I just painted the bottem of the boat 3 weeks ago, so underneath is ready to go. as for getting pictures I won't be able to do that since the boats back in the water now. Underneath is clean and smooth though. The prop looked ok, its part # is 391535. I have been trying to get a nice day to go out and get some testing done for you, but all it does is rain here. Hopefully on the weekend I will get out on it.

 

[hwsiii]

Re: Help With Propeller Selection

As soon as you get the numbers I will be ready.

H

 

[seawise]

Re: Help With Propeller Selection

well this is fun. The boat would not get up on a plane and it was doing 4100RPM and would go only 12knots so i put a new propeller on incase the hub was slipping on the old prop. Same thing though maybe gained 100RPM but the current was running in my favour. Then i went back to the dock started checking everything out really well, found a spark plug wire that was arching to the block. So i put brand new plugs, wires, cap and rotor made sure the timing was dead on went back out the boat will go 4500-4600RPM @ 12.5-13knots and still wont plane. I am thinking maybe its an engine coupler starting to go. anything else you guys think ive overlooked?