Reducing prop diameter to increase revs - Anyone know??

[swamp23]

Does anyone have any experience reducing the diameter of a prop to increase revs? Is there a calculation to work out how much to reduce diameter by to gain revs?
I have a Johnson 225HO and have recently acquired a Signature 4 blade 30p 13.5" diameter that is 400 to 500 revs below where i would like it. Short of replacing this for a smaller pitch is there a formula to work out how much diameter i would need to reduce by to get this gain? eg 1/4" reduction equals 200rpm.
Obviously i don't want to go to far with this and would make small changes but any info would be appreciated as to roughly where to start so i don't remove too much initially and find the revs have increased too much. I will be aiming to keep the blade shape as design intended just removing some length of the tips of the blades.

Thanks in advance

[vnemous]

sounds like the prop has already been clipped. stock is 14.5 diameter I believe. You probably already lost a lot of lift.

[outasite]

In general pitch is speed,and diameter is ability to move weight 13.5" on a 225 is already on the lean side for pushing much.You will reach a point (soon) when the slip rate will go nuts.The revs will be there,but not moving enough water.

[dnelson964]

I would not touch it anymore at this point. Best to just sell it and buy another prop. You might look into a faster style prop also, perhaps a Turbo TXP-4 28 pitch. JMO

[wicheckmate]

Just take a little pitch out of it .... Should up your revs a bit

[swamp23]

Thanks for the info everyone. I'll look at replacing it rather than reducing the diameter.
How does the Turbo TXP-4 28p compare to a trophy 28p? I have run trophy 's before and have access to try a 28p to test.

[vnemous]

Boat, motor info will help

[whipper]

Thanks for the info everyone. I'll look at replacing it rather than reducing the diameter.
How does the Turbo TXP-4 28p compare to a trophy 28p? I have run trophy 's before and have access to try a 28p to test.


As Vnemous said knowing your setup helps but any time you can try a prop do it! Theres an old saying ya cant try to many props. Trophys are generally really {good} props. On some hulls {great] I would say theres not to many they dont do well on for what they are. Some times depending on were your prop shaft height is already as you go {up} you gain rpm. So as mentioned whats your prop shaft height in relation to the pad,What hull is the motor on, Are there mods to motor and whats the motors rpm limit,also whats your setback. In your sig swamp 23 Im assuming the hull is a 23fter? if thats the case i can say with pretty good certainty that a 30 pitch is a big wheel to get the most out of a 23fter with any 225.so as mentioned a little more info helps with better answers. :cheers:

[swamp23]

Ok being based in New Zealand the boat is a locally built 19ft Kevlar vee bottom ski boat as per picture.
376880
The engine in a standard 2001 225ho with a bobs nose cone being the only change from factory, the class rules here dont allow us to modify the engines so set up and propping is important.
I can spin a SC3 30p and a ET 30p to 6300rpm at 83mph, the signature 4 blade 30p did 82mph at 5500rpm but was pretty sluggish to get there. I would ideally like to get the rev around that 6000 mark but maybe this is not possible with the prop design and boat combination.
I have a 6" set back hydraulic jacking plate that i run approx 6.5" high but will measure to confirm.
I have also triple checked the prop is definitely 13.5" diameter and is like new so confident hasnt already had diameter reduced. I have a 26p trophy which maxes out at around the 77mph mark.

Any thoughts and set up tips appreciated!!!

[phillnjack]

13.5 pitch is what gets used on 70hp triples, it must look tiny on that motor.
dont trim /cut any smaller or youl never sell it , take a bit of pitch out .

[whipper]

Ok being based in New Zealand the boat is a locally built 19ft Kevlar vee bottom ski boat as per picture.
376880
The engine in a standard 2001 225ho with a bobs nose cone being the only change from factory, the class rules here dont allow us to modify the engines so set up and propping is important.
I can spin a SC3 30p and a ET 30p to 6300rpm at 83mph, the signature 4 blade 30p did 82mph at 5500rpm but was pretty sluggish to get there. I would ideally like to get the rev around that 6000 mark but maybe this is not possible with the prop design and boat combination.
I have a 6" set back hydraulic jacking plate that i run approx 6.5" high but will measure to confirm.
I have also triple checked the prop is definitely 13.5" diameter and is like new so confident hasnt already had diameter reduced. I have a 26p trophy which maxes out at around the 77mph mark.

Any thoughts and set up tips appreciated!!!

Two things come to mind. One is getting your hull to run dryer. Have you tried running the prop even to above the pad? the more you go up the more rpm you'll get. You will for sure loose rpm going to a 4 blade from a 3. Another thing is to blunt the bullet on the Bobs cone. it should be round like the end of your finger. The idea behind this is to loose suction from the sides of your gear case. Kinda like the mirror on a wet glass table effect. Its hard to pull the mirror up because of suction. The water with a blunter bullet hits then deflects away from the sides then comes bak in at speed just before the prop so you dont loose bite. Also wet sand gearcase 400 grit sand paper. Some believe primer wetsanded is the best? ive tryed both. For sure not smooth that gets back to the suction thing. Drag racers do this a lot of them anyway maybe not all. Also the wetted area of the back of the hull wetsanded cross ways chine to chine 400 grit. Not sure how big your skeg is but an Allison cut on the skeg will also reduce drag. 6.5 high on the jack means nothing compared to prop shaft hight in relation to the pad. Ideally you want to be as high as possible before loosing water pressure trimed before blow out. What i do is start at even with the pad and run wot trim then trim untill my rpm goes up a little and the mph does not. Back off then jack it up 1.4 inch and repeat. ill do this untill Ive got the highest mph before rpm goes up and speed does not always paying attention to the water pressure of course because that could be the limiting factor before Hight and RPM/MPH are.

This prop may like to be run higher than the ET to be more efficient? Anyway short of a different prop and pad mods thats all I got. I suppose you cant run to dry with less weted surface as you need the keel in a ways to help with the pull of the skiers around corners? A 28 trophy will do better than 82 mph at 6000 rpm. Also if you reduce the dia your also reducing the props carying ability with a load. Not a good thing in most cases either. It almost looks like the 14 to 14.5 dia props are fastest because of the better lift the larger dia gives you? In the trophys If i remember corctly the 28 is a larger dia than the 25 to 27 pitch ones? id have to look to confirm. Seem to remember that. Thats why the 26 may have been a dog at 13.5 dia also you need more pitch for your gear ratio and two more dia to get the stern to lift higher for less hull drag?

[swamp23]

Thanks Whipper, great info.
I've already blunted the nosecone, filled in two of the five pickup holes and sanded entire gearbox so i will look at my running heights this weekend and see if i can pickup speed. The 6.5" i referred to was from the pad up to the cavitation plate so will measure how that relates to the prop shaft height.
Thanks again for you thoughts

[GregAdams]

Do not cut your skeg. Are you running SMOC ski racing, or Stock Mono circuit racing?

[whipper]

As Greg is alluding to either the skeg cant be modded and or when skiing the reduced weted surface area of the hull having more skeg will help with the pulling of the stern at speed around corners. One things for sure a god prop when skiing I find makes a big difference also. Generally a prop with a little more tip cup will reduce slip numbers also a bigger dia not smaller. So with a smaller dia prop tip cup might help? weight distribution also. It sounds like you've already tried everything.:D