Looking to change Power for speed.

genesis426

Recruit
Joined
May 1, 2010
Messages
2
Hey all, new to this forum but not new to boating forums. Looking for advice as the local dealers are unable to help. I have a 1974 17ft Caravelle Split Hull Fiberglass "Speed Boat" with a 74' Mercruiser I/O 140. I believe it is with the alpha I sterndrive. Mainly we use this boat for tubing and skiing behind and have since I was a kid. More recently we have had a need for an increased speed as it gets used for fishing and slalom skiing. Currently I have a three Blade and Four Blade prop for it. Both are 14" and one it 17p and the other 20p I believe. I am planning on taking this boat to Canada in a few weeks and am seekign advice on finding a prop that I can put on to possibly gain a few mph's or to drop the rev's down. I understand that it isn't necessarily as easy as it sounds but even an extra mile an hour or so will help. Currently at WOT (approx. 4800rpm's) i can do about 36mph. looking to gain some speed or keep ths speed at lower revs to save fuel. Any Advice on a good prop. Or, more importantly does anyone know the operating rpm range for this setup so that I could have a prop built.
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Looking to change Power for speed.

Genesis, my information shows your recommended range should be between 4,200 and 4,600 RPM and a gear ratio of 1.98:1.

You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057


If you would prefer to not go to this much trouble there are many other people who will come along and gladly help you.

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the maximum recommended HP for your boat
3. Number of people and gallons of gas normally on boat and what do you use the boat for
4. Year, make. manufacturer and model of motor and recommended Wide Open Throttle (WOT) range
5. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
6. Anti-ventilation Plate height above keel of boat if it is an outboard in inches, use a straight edge under the keel and sticking out to the anti ventilation plate for a reference
Motor Transom Height
AntiventilationPlateStraightEdge-2.jpg

7. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating.
RPM _________ Speed ______ GPS ______ No. of people ______ Gal. Gas ________
8. Make, model, diameter, pitch, number of blades and whether SS or aluminum prop and does it have a hydrafoil, dolefin or trim tabs
9. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way by just pushing the throttle at the helm and not at the carburetor, checked compression, and looked at the plugs and checked spark as well as timing and advance, all of the foregoing could be the reason your prop is not attaining full RPM
10. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations and does the prop show any damage that you can see

REMEMBER, The numbers I give you will be NO better than the information you give me. I use a computer program I designed to find the correct pitch and then knowledge to pick a Better prop design for your boat.

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.



H
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Looking to change Power for speed.

If anyone can help you its H.
Your boat will use about 14gph(gallons per hour) at wot(wide open throttle).
whether turning 4200 or 4600 rpm.The idea is to produce the most speed for a given throttle setting.wot isn't the way to save fuel.finding the best cruising speed
is the way to save fuel.Generally a 4 blade prop will get you on plane quicker(a fuel saving)keep you on plane with less power(a fuel saving)yet in some cases produce
close to maximum top speed.The best cruising speed is usually between just on plane and about 600-800 rpm higher.Only way to know the ideal is to test test test
or get a flow meter.If you get H all the info he usually can help with a prop and tell you the expected results.
 

genesis426

Recruit
Joined
May 1, 2010
Messages
2
Re: Looking to change Power for speed.

To the both of you, thank you for the information provided thus far. After reading more of the threads I discovered that I needed to provide more information. I will be sure to get that information out within the week. I am in the middle of redoing the wheel hub assemblies and brakes on the boat trailer and once its done I will be able to get out and get speed and wieght information. I had a feeling I had been somewhat close with the prop selection thus far...after unfortunate trial and error.
I again, thank you guys and am hoping to get the info needed and posted by thursday.
 
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