Prop help...and yes, i read the sticky...

[mrdjflores]

here's what we got going on....
answers from sticky thread:
1) low top speed (approx. 30 mph)
2) aluminum 3 blade prop (michigan wheel i think)
3) 14"dia x 17" pitch
4) 4k rpm @ 30mph (not gps confirmed)
5) Merc 888 (MR/Alpha ???) 1973, 188hp
6) 1973 seaRay SRV220 OV, 22' bow to transom (maybe 24' LOA), have seen it listed to be 3800lbs dry weight ???

I am not looking for a quick hole shot, but would like more top speed (of course).
I am in the process of building a new motor for it (keeping it under 300hp) and will more than likely be buying a new SE106 complete drive (1.47 ratio)

still looking to be around 4000 rpm cruising...change ratio??? change prop size??? 3blade or 4 blade???

looking for input

thanks,
Dan

 

[Bondo]

Re: Prop help...and yes, i read the sticky...

Ayuh,... Dan,.. Finish the motor rebuild,+ whatnots,..
Then,.. Run what you have to determine a Baseline...
You'll need to reprop, After the rebuild, to achieve the recommended rpms@wot...
You should shoot for 4600rpms@wot...

 

[mrdjflores]

Re: Prop help...and yes, i read the sticky...

thanks for the info...in your opinion, should i want peak HP at 4600, or peak torque?

 

[Bondo]

Re: Prop help...and yes, i read the sticky...

Ayuh,... Horsepower is nothing but a mathamagical equation derived from the measured Torque output....

The most Torque at 4600rpms will be the basis for the HP at 4600rpms...

You want to buy a cam that builds power down Low, up to 4600rpms...

 

[smokeonthewater]

Re: Prop help...and yes, i read the sticky...

The cam should also have minimal overlap to prevent waves hitting the back of the boat from causing water to surge through the exhaust into the engine
The tech guys at summit racing can help with your cam selection and I have been very satisfied with their cam sets. 800 230 3030

 

[mrdjflores]

Re: Prop help...and yes, i read the sticky...

i'll keep a note on all this. thx

 

[hwsiii]

Re: Prop help...and yes, i read the sticky...

DJ, I ran some numbers on your boat with information that i found, I found that your motor has a recommended RPM range between 3800 and 4,200, and I also found that your gear ratio is 1.5:1, please verify this.

If this is true, I show you have a prop Slip of 30% and that is very very high for a small boat.

Prop Slip

H

 

[Don S]

Re: Prop help...and yes, i read the sticky...

The cam should also have minimal overlap to prevent waves hitting the back of the boat from causing water to surge through the exhaust into the engine

The overlap has nothing to do with what happens when waves hit the back of the boat. The exhaust shutters (aka flappers) prevent that. What I think you are referring to is reversion, where the valve overlap will cause the water to get sucked back into the engine instead of going out the exhaust.

 

[hwsiii]

Re: Prop help...and yes, i read the sticky...

Dj, just to give you a feel for the differences in speed that can be attained by changing props for a lower prop slip I charted 2 different props with prop slips of 10% and 20% to show the differences in speeds attained by lowering your prop slip from 20% to 10%.


Prop Slip Percentages Graph

I am using the corresponding color for the text to explain what the chart is showing us.

The 19" Pith prop with only 10% prop slip is actually able to attain 54 MPH at 5,000 RPM.

The 19" Pitch prop that has 20% prop slip is only able to attain 48 MPH at the same 5,000 RPM, because the prop does not have enough blade area ratio or the correct blade geometry or a combination of the two.

Most of the time that people come here looking for a better prop for their boat they have very little knowledge on how to setup an outboard motor for maximum efficiency and thus are not getting maximum performance from their motor, much less their prop.

By raising the motor up higher on the transom you are able to get more RPM and thus able to turn a prop with more blade geometry and lower their prop slip to attain the higher speeds.

Or if the motor is as high as it can be raised you can change to a 4 bladed prop in the same pitch and attain the same speed or better, but have much more low end pulling power, with a lower planing speed, better acceleration at all RPM settings and a much better ride with less vibration.

And when you have an inboard outboard like you do you can change to a 4 blade prop in a lower pitch to attain the correct RPM for your motor and also drop your prop slip to a much better level and have all of the above advantages that come from the 4th blade.

H