OUTBOARD PROP

[hardy]

hI,
I have just re-engined to a Yamaha 50 high thrust 4 stroke. I have it on a Hardy Regatt, semi planeing hull. It should get me on the plane and acheive 6000 revs.it does neither.It is fitted with a standard prop as supplied,marked 13.k.
Has enyone any knowledge on the subject of changing the prop size and to what.I mention it to the supplier of the engine near Chichester and their eyes glaze over!!
Many thanks

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[Happy1]

Try the link below and download the small prop calculation programme it is FREE. As it states it is for you to use at your own risk, they will discuss the results with you but you have to pay for the time. There is nothing to lose by doing a calculation on the free software, I have done it as I am interested in studying props in depth.

<A target="_blank" HREF=http://www.castlemarine.co.uk/pitch.htm>http://www.castlemarine.co.uk/pitch.htm</A>

Good luck, and let me know what you think,

<hr width=100% size=1><font color=purple> "You only see what you recognise, and you only recognise what you know" <font color=purple>

 

[MickJ]

I had the same with a Yamaha F50 and a 13" prop. Only 3500 at WOT and it wouldn't plane. The dealer wasn't interested either.
I changed it down to a 10" prop and got 5500 at WOT and it got onto the plane at about 4200rpm.
I thought the high thrust models where specifically designed to be used with low pitch props anyway?

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[andyball]

The high thrust versions usually swing a larger diameter ,lower-pitch prop....often with an extra set of exhaust passages in the hub facing forwards...to improve water flow in reverse gear. Sometimes (not always) they have a different gearbox ratio, to spin the prop. slower for the same engine rev's...sometimes a larger gearbox from the next size up engine.

As MickJ suggests, you'll need a lower pitch prop, maybe a smaller dia. too, depending on what's actually available to fit.

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[hardy]

Many thanks to all who have answered.will try prop calculations and check out available smaller props, expensive trial and error me thinks!!

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[BarryH]

Give one of the specialist companies a call. Some like Steel Developments ao Lancing Marine or Poole propulsion. They give you a better idea of the size required. They'll ask for some info like waterline lenght hull wieght and gearbox ratio, but it will be less hit and miss. Hope its of some help

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[MickJ]

What are your revs at WOT? I was told to expect 2/300 extra r/min's for each 1" pitch drop, but got considerably more (must be because I was so over propped in the first place the engine couldn't get anywhere near its power band).

Dont want to worry you, but you can destroy s/plugs if you are badly over propped, as I did with my engine when only 10 hours old. No probs (yet - touch wood) since changing props.


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[jfm]

13inch pitch sounds too high. At full revs that would be pushing the boat at 22-23 knots, but that's way to much, you will never get that speed with that rig, 15knots more like, which implies about a 9-10inch prop

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[hardy]

whats WOT. I have no idea about such things new to it all.

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[BarryH]

Its one of those internet things. WOT= Wide Open Throttle, IMHO and FWIW

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[hardy]

AHH!
5500 revs wide open speed can vary from 11 knots to 17 knots depend on sea state and weight onboard , struggles to get on the plane.
Used to get 17-18 knots out of an old 50 Evinrude 2 stroke.

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[MickJ]

I might be mistaken, but I thought high thrust outboards had a different gear ratio and are designed to be used with low pitch props to give plenty of low speed torque, specifically for working boats or sail boats?

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[andyball]

That's what they're usually sold for.

But some have the same gear ratio, just a beefier lower unit.

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[hardy]

It is great for manouvering at slow speed, but it was sold to me as the prefered engine for the Hardy Regatta to enable planeing and top speed,When you go to these people for advice you assume they know what they are talking about.Being in the medical proffession people assume I will do whats best, I have this failing where I assume others do the same.
I have just done the calculation on propcal.it says I need rec.prop 0.92 /pitch ratio o.oo /me thinks I may have done something incorrect as at that spec. I should attain 18.96 knts.(I WISH)

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[Col]

Have you tried asking Hardy themselves? Would have thought they have tried most engine combos on their boats.

<hr width=100% size=1><A target="_blank" HREF=http://www.arweb.co.uk/argallery/colspics> Cols Picture Album</A>

 

[andyball]

certainly not the choice for top speed....what's the reduction ratio on your new engine?

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[hardy]

Have tried Hardy as the boat is old and no longer in production they have not replied.
reduction if that means gear ratio 2.30(930/13)
Once again thanks to all trying to advise

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[andyball]

A std. F50 engine wd be 1.85:1

I would call yamaha marine technical at ? & explain the problem...have found them very,very helpful in the past...& they'll know exactly what prop's are available. Don't have the number, but try 01932 358000 for starters

They might ask why you bought that model if speed is important, so do tell them which yamaha dealer recommended it,knowing the boat & your requirements.

If the right prop's can be had, it can be fixed (some suzuki 4-strokes use 2.6:1), but it does depend on a suitable prop being available.



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[hardy]

Hi again will try that number. the engine is I quoteFT50CETL. does that make a difference. the gear ratios you quoted would they be better than I have.
many thanks

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[andyball]

your engine is the high-thrust version, usually pushed at heavy displacement boats, not for speed .....have heard people doing ok with them on ribs etc (despite what I just said), but maybe with more power to spare than you have?.

Ratios quoted are simply the standard F50 (non-high-thrust) =1.85:1....not better/worse, but wd need propeller to suit.

eg: my suzuki is 2.6:1, so prop turns quite slowly for given engine speed, but large dia, ,large pitch prop makes up for this.

<A target="_blank" HREF=http://www.yamahamotor.com/accessories/otb/propellers01.html>http://www.yamahamotor.com/accessories/otb/propellers01.html</A>

Yamaha shd know if a suitable prop can be bought to fit your engine...& I hope would be concerned that you were recommended high-thrust version if speed was important to you.

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by andyball on 15/04/2003 21:07 (server time).</FONT></P>