General questions on pitch vs RPM

[Jacob J]

Hello all

I have two questions:

1. Will a boat always perform better running at highest recommended RPM with a lower pitched prop?

2. Is there any situations where a prop with a higher pitch and the engine running at lower RPMs will perform better?

If I read 5000 rpms on my boat when I get the tachometer installed and then change the prop to one with a lower pitch and get the engine up on 6000 rpm as it is maximum recommended to, will that mean I get more top speed, altho it is a lower pitched prop I install? The point is, that I see no need to change the prop if I have to run the boat at 6000 rpm and get the same speed out of it as I do now running 5000 rpm (hypothetically speaking).

My boat (16 foot) is balancing on the edge, where the power is limited (25 hp) and the weight is high (520kg, without me in it). I run 24 km/h with some gear and one passenger more in the boat, but when I am alone in the boat it runs 36 km/h. So 120 kg makes that speed difference. Tomorrow I will install a tachometer to see what WOT rpm the boat is running at when going 36 km/h and 24 km/h to see if I need to change the prop. I know there are more to it than that, but maybe I can get some more advice on that subject from you guys on here later on.

Thanks in advance

/Jacob

 

[steelespike]

Re: General questions on pitch vs RPM

1. A motor will have more power if propped closer to its max rated rpm,Not necessarily the fastest.
Sometimes increasing pitch will lower rpm and yet increase speed.Depends on where you start from.
2. A higher pitch is possible could produce higher speeds at lower rpm than a similar lower pitch prop at the same rpm.
Trying to put a boat into overdrive doersn't work.
Increasing pitch reduces power. So the boat will hole shot slower, fade faster as you add weight, may be fussy when trying to run at reduced throttle. The motor may lug if it falls out of the wot rpm range.
Propped close to the max rpm will produce quicker hole shot,better weight handling, will stay on plane easier at reduced throttle
and probably fastest.
Tell us more about your setup. Brand,year and material of the boat,Tiller or full dress,remind us of the weight. Brand,year hp.
and prop size?(near base of blades or under the prop nut). Wot rpm and gps speed lightly loaded.

 

[Jacob J]

Re: General questions on pitch vs RPM

Okay here is my setup:

Boat: Alaska 500 (16 foot)
Year: unknown
Material: Fiberglass
Tiller or full dress (not sure what this means), but I guess its how I steer the boat. I steer by turning the motor.
Weight: 520kg without any person in it

Motor: 25 hp Mercury bigfoot 4-stroke from 2001
Prop: 12 x 29
Gear ratio: 2.42:1

WOT rpm: comming soon, but guess its in the 5000-5500 region.
GPS speed lightly loaded: 36 km/h, GPS speed with one more person on board is 24 km/h.

Any other information needed?

Ive read that putting the engine 1 or 2 inches higher on the transom could get more speed. Right now it is mounted close to flush with the bottom of the boat, but still an half an inch under as far as I remember.

 

[Sea Rider]

Re: General questions on pitch vs RPM

Some issues:

Full HP is reached near max rpm at wot, if you get full max rpm at wot with you solo, much better, with added weight rpm will drop accordingly. When installing tach check max rpm with you alone and then with second boater to see how many rpm your engine is losing.

On a light boat standard factory props usually delivers near max stated rpm at wot for a given portable engine, in your case having a heavier boat should be around middle min-max safety parameters. Once tach is installed, will know for sure. To optimize engine performance while sitting on transom will need several tech issues to take into account, start with wot rpm and then will work on it.

Happy Boating

 

[Jacob J]

Re: General questions on pitch vs RPM

Okay thanks for the tips. I will install tachometer and check with me solo and then with a second boater. Should there be any other gear in the boat (more weight)? Usually we have 30 kg of gear in the boat at least, should that be taken into account or is that the spill in rpms I will have to accept to keep the engine out of harms way?

 

[steelespike]

Re: General questions on pitch vs RPM

!2X29 Isn't prop size I think it is your gear ratio expressed in gear count, 29 divided by 12 = 2.42.
Your prop size is near the base of the blades or under the prop nut.

 

[Jacob J]

Re: General questions on pitch vs RPM

Okay I'll have a look again. Brb

 

[Jacob J]

Re: General questions on pitch vs RPM

Okay, only this number is present ; 48 816704A40 and there seems to be a faded 8P after a space. From googling that it says that it is a 10.5 x 13 prop.

 

[Sea Rider]

Re: General questions on pitch vs RPM

Which brand and HP is your engine ? If being a mechanical tiller steering, float boat alone, adjust trim so to have engine perpendicular to water level, if boating solo, move your weright bit forward. If being 2 boaters, move one up front, go full throttle, check both wot spins. 30 + Kg extra gear it's ok for testing. Must have a well ballanced boat riding near paralell to water surface for more accurate max rpm readings.

Happy Boating

 

[jestor68]

Re: General questions on pitch vs RPM

The 8P means it's a 8 pitch prop.

 

[jestor68]

Re: General questions on pitch vs RPM

The motor's recommended WOT rpm range is 5000-6000 rpm; with rated hp @ 5500 rpm.

For best all around performance(not just top speed), you should prop to achieve at or near max rpm with a light load; so that when you do add the extra weight of more people/gear, the rpm should not drop below the hp rated rpm.

If you prop just for top speed with a light load, it will fall flat on it's face when you put that extra person or two in the boat. If you had a bigger motor, you would have more leeway; but that little 25 is going to be working it's heart out all the time just trying to push you alone in that boat.

 

[steelespike]

Re: General questions on pitch vs RPM

Boat alone is 1100,lbs. + 157 for the motor 200 for driver,36 lbs for gas, 50 lbs for gear.
I think we are going to need an 11" pitch. the rpm will tell.

 

[Jacob J]

Re: General questions on pitch vs RPM

Okay are you sure that 8P means a pitch of 8? A pitch of 11 will be an increase in pitch and that means lower rpms. I think higher rpms are needed. I will update on the rpm readings when I get that tachometer working.