Propellers

[Racingfrank7]

Hi !
A work mate has just asked me if changing the prop on his outboard would be a cheaper alternative to buying a bigger engine ( he is wanting better performance ) we both know the proper answer is buy the bigger outboard, but then it got me thinking due to hearing other people talking at the marina, saying they had changed there prop and its made there boat plane ( wouldn't before ) which leads me to ask how you would work out what pitch diameter and the difference between 3 blade props and 4 ?
Laymans terms please !

Ps he only has a little honwave rib and a 10hp Honda 4stroke which all came as a package new about 2years ago.
Cheers.

 

[scottie]

All propellers are a compromise

You could ask latestarter1 but he would most likely tell you to speak to ken wood about egg whisks

You can't increase power by changing a prop any more than you can increase power in a car by changing gear but you can get the best performance in some situations by doing so
Ie drop a gear to get acceleration in a car
Simply and it is very simple diameter is power and pitch is speed within a small window.
by altering either you can adjust the required performance

However to calculate correct sizes you go to the experts and even they will only get with the ballpark and then it's trial and error experience black magic etc. to get perfect and that will only be for a choice of top speed,acceleration,economy,towing a skier etc etc

WEIGHT will be biggest factor in this case!,,,



You may also be limited by what props are offered by the manufacturer when it comes to outboards

Have fun!

 

[landlockedpirate]

The Honda's are excellent engines and inflatables, but the motors are very heavy. It may benefit from a smaller prop, a dolefin or trim tabs. What Honwave is it ?

 

[Racingfrank7]

All propellers are a compromise

You could ask latestarter1 but he would most likely tell you to speak to ken wood about egg whisks

You can't increase power by changing a prop any more than you can increase power in a car by changing gear but you can get the best performance in some situations by doing so
Ie drop a gear to get acceleration in a car
Simply and it is very simple diameter is power and pitch is speed within a small window.
by altering either you can adjust the required performance

However to calculate correct sizes you go to the experts and even they will only get with the ballpark and then it's trial and error experience black magic etc. to get perfect and that will only be for a choice of top speed,acceleration,economy,towing a skier etc etc

WEIGHT will be biggest factor in this case!,,,



You may also be limited by what props are offered by the manufacturer when it comes to outboards

Have fun!

Thanks for that I will tell him.

 

[Racingfrank7]

The Honda's are excellent engines and inflatables, but the motors are very heavy. It may benefit from a smaller prop, a dolefin or trim tabs. What Honwave is it ?

I think it's a 3.2m and it's got a 10hp 4stroke on it, he says "it's ok but a bit sedate, it planes ok with 2people in 4 you have no chance ! but it's not quick by any means, it's a bit flat it don't really lift" ! I clocked him at about 13 knots last season 2up with some fishing gear on board, it's rated on plate to take a max 15hp but after reading thread on here about big engines on ribs and seeing some locally he is toying wi getting a Honda 4 20hp !

 

[[2068]]

There's no way 10hp is going to get 4 people planing, whatever propellor he fits.

5hp just about gets one FB (me) planing, and thats in a much smaller boat.

25-30hp would do the job, but it depends very much on weight of passengers.

 

[landlockedpirate]

I think it's a 3.2m and it's got a 10hp 4stroke on it, he says "it's ok but a bit sedate, it planes ok with 2people in 4 you have no chance ! but it's not quick by any means, it's a bit flat it don't really lift" ! I clocked him at about 13 knots last season 2up with some fishing gear on board, it's rated on plate to take a max 15hp but after reading thread on here about big engines on ribs and seeing some locally he is toying wi getting a Honda 4 20hp !

I agree with everyone else, a new prop wont improve the performance, a doelfin may help with getting on the plane but with limited power it will hurt top speed.

I had a 3.2 a few years ago with a 2 stroke 15hp on it. Planed easily with 4 up and did over 20kts with 2 people.

I agree with more hp, but not in the case of the air rib, I think a 15hp is as much as you should go. At speed when fully loaded the floor started to flex and become slightly concave, it wouldnt take much more power or weight to invert the floor IMHO. The Honwaves are excellent boats, either ditch the heavy 4 stroke and buy a lightweight 15hp 2 stroke, or sell the lot (you will get really good money on ebay for the boat) and go bigger and more powerful. If portability isnt an issue go for a hard deck with a 20-30hp, good for 30kts.