help me choose a pitch

[gbigskin]

i have a 16'7" seamark (whaler shape hull). wt. 760lbs
2007 suzuki df50. wt. 235lb rpm range 5800-6500
people and gear. wt. 500lbs
15 gal. fuel

11 5/8x12 suzuki 19.5mph@4900rpm wot 28.2 6250rpm
11.6x11 solas 14.8mph@5000rpm wot 25.0 6600rpm
11.1x14 solas 20.5mph@4800rpm wot 28.5 5800rpm

i would like to re-pitch one prop to get a cruising speed of approx. 20mph and lower my current rpm at this sporeed.
which prop would you suggest i re-pitch and to what pitch to achieve this or is it possible?

 

[1080tommy1]

Re: help me choose a pitch

To me it seems that the 11 5/8x12 is the prop to work with because it gives the best wot rpm. The 11 pitch just seems to be too high at wot and I have a 40 hp and will be running a Solas 3X11 1/8X13. The 14 pitch is really at the extreme low end of your rpm range and seems to be lugging at wot. The 12 is almost where you want to be and if it is not cupped, that may be the thing you need to get a mph or two and she is right in the upper part of your rpm range. Good luck and I hope you get it dialed in. I am sure you will get some good feedback here.

 

[steelespike]

Re: help me choose a pitch

As suggested the 12" prop is doing a good job slip at 6250 is about 9% a slightly high normal.
The 12 has 20% slip at 4900 a reasonable number.
Is the anti vent plate(just above the prop) about even with the bottom of the transom?
Have you tried trimming down to make it easier to stay on plane?Have you tried moving weight forward?
Are the battery and gas in the stern?
Marine engines need to rev freely within their recommended range.And its obvious this motor is intended to rev.

 

[gbigskin]

Re: help me choose a pitch

the suzuki prop has been cupped and the vent plate is at the bottom of the transom. solas props are not cupped. i was thinking of changing the 11.6x11 solas and putting a 12 pitch and adding cup before i ask for help or putting cup and changing the 11.1x14 to a 12pitch thinking the smaller dia. would spin up faster. opinions on this are appreciated.

 

[QC]

Re: help me choose a pitch

i would like to re-pitch one prop to get a cruising speed of approx. 20mph and lower my current rpm at this sporeed.
which prop would you suggest i re-pitch and to what pitch to achieve this or is it possible?

Why isn't the 14 perfect for you? I understand that it's at the bottom of the WOT range, but it's in spec and best at cruise speed. I am not sure why you need to do anything? This is for discussion, but it is also what I would do. If she planes OK, I don't think you're lugging her at an efficient planing speed like 20 MPH

 

[1080tommy1]

Re: help me choose a pitch

I think being at the bottom of the WOT range is not good because if any additional weight is placed into the boat, the rpms will go down further. Depending on weight, that could result in a lot of rpm loss and then the motor would be considered to be lugging. I think the 12 would be perfect with a WOT right in the upper end of the spectrum but also the cruising rpm would lend itself to very good fuel economy as well. Now if cupping was added to that prop, it may even yield better results but that seems to be the best option if any weight will be added under power. I guess that 14 could be worked on as well to better suit it for use on this hull, but why spend the dollars on that if you already have a 12 pitch that yields .5 off of your desired cruise speed and at WOT is perfect. I guess it's just a personal thing now and I hope you get the numbers you are looking for. I know it can be challenging.

 

[QC]

Re: help me choose a pitch

I agree if his weight as shown is not typical. If the goal is a low RPM cruise, for whatever reason, then it seems good as is. That was my only point.

 

[1080tommy1]

Re: help me choose a pitch

I agree. Always interesting how a post can give much good knowledge on different opinions. Especially with this topic because so much depends on the individual situation and how the given boat is set up.

 

[steelespike]

Re: help me choose a pitch

If you force the motor to a lower rpm by fiddling with the prop you will likely change the throttle setting to attain the lower rpm.
Throttle could be more wide open than with a a prop that lets the motor rev.

 

[gbigskin]

Re: help me choose a pitch

Why isn't the 14 perfect for you? I understand that it's at the bottom of the WOT range, but it's in spec and best at cruise speed. I am not sure why you need to do anything? This is for discussion, but it is also what I would do. If she planes OK, I don't think you're lugging her at an efficient planing speed like 20 MPH

i don't think the 14 turns enough rpm's. am i wrong in thinking that this smaller diameter prop pitched down to a 12 would get the rpm up closer to max of 6500 and increase the top speed and also let me cruise at a lower more fuel efficent rpm? also i did not understand steel spike's last post. could you please phrase it another way if possible so i could understand what you are telling me.

 

[1080tommy1]

Re: help me choose a pitch

I think that with less pitch, the motor can rev more freely. In turn, this will give you the cruise you want with a more appropriate rpm level because if the motor is lugging, you need more throttle to get and stay on plain. For example, the 14 appears to have the exact specs you want as far as speed, however; the motor is running very low rpms which is not allowing the motor to rev freely. I think that the 12 pitch is also about where you want to be as far as speed is concerned but the motor will be much happier. I think that changing the pitch would also work but going from an 11.1 which I think translates to an 11 1/8 to an 11 5/8 should not affect rpms that much. I read somewhere that diameter does not affect rpm nearly as much as pitch so that is why personally, I would go with the 12 pitch. Less time an money spent, more time on the water.

I am just completing re-propping and I had little to no reference so I know this can take some trial and error. Hope you get her dialed in where you want.

 

[gbigskin]

Re: help me choose a pitch

If you force the motor to a lower rpm by fiddling with the prop you will likely change the throttle setting to attain the lower rpm.
Throttle could be more wide open than with a a prop that lets the motor rev.

amazing what reading something after a good nights sleep will do. after reading your post again this morning i understand what you were telling me regarding rpm in relation to throttle.

 

[Philster]

Re: help me choose a pitch

Personal goals matter when deciding the best answer.

It it were me, I'd see the 11 5/8x12 suzuki as falling too far short of the max RPM and would want it to be about 100 RPM of the limit, then you'd know that you could carry a bigger load or use it in harsher/hotter conditions.

When it gets hot and muggy out, the RPM posted will be worse, too, because power is lost with summer heat. So, anything scraping the bottom of the RPM range now will be below it come summer.

Ideally, I'd get that prop tweaked to turn some more RPM and call it a day. Heck, the one that is over-revving now -- I might choose that one. The prop churning the lowest RPM now? I'd shelf it.

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