Teach me effects of prop selection

[Old Crow]

I have two aluminum fishing boats

2004 Lund Pro Angler 16
16' 6" 1045lbs
2003 Yamaha F75TLR
13.5 X 15 Yamaha aluminum 3 blade prop
WOT 5600-5800rpm at 36mph

2004 Lund Angler SS
17" 2" 1115lbs
2004 Yamaha F115TLR
13 X 19 Yamaha aluminum 3 blade prop
WOT 5800-60000rpm at 43mph

Comparing WOT runs between the two boats is comparable as far as amount and weight of extra gear on the boat

Overall power to weight ratio should benefit the larger boat but the small boat seems to get out of the hole better.

What effects does prop diameter have on the boat? Notice the small motor has a larger prop
I would like to help the out the hole performance of the larger boat without increasing it's idling speed; at the same time maybe increasing top speed

 

[steelespike]

Re: Teach me effects of prop selection

Pitch is the primary consideration in prop selection. lower numbers are like the lower gear in a car,higher numbers like the higher gear in a car.When selecting the best pitch for your motor/boat conbination you are in effect picking a single best gear for your and the motors needs. Weight distribution,bottom shape gear ratio trim angle all affect hole shot.Both motors seem to be propped very nicely.They must be able to operate within there designed wot rpm ratings.The motor weight difference alone could be affecting hole shot.

 

[Old Crow]

Re: Teach me effects of prop selection

Will a larger diameter prop with the same pitch put more load on the motor?
I would think the larger blade would grab and push more water, even if it was of the same pitch.

I always trim all the way down during hard take offs.

Boats are pretty much identical. Smaller one is a tiller and the other has single side counsel.

Weight difference between the motors is only about 30-35lbs.

I just picked up the larger boat and it came with about 30 gallons of gas that was put in the tank last summer. I should have sucked it out but decided to just use it. I'm not putting long heavy loads on the motor till I get some fresh gas in it. When I get down wind of the motor while idling it smells rich

 

[Silvertip]

Re: Teach me effects of prop selection

I don't understand why, but people get all wrapped up with "diameter" when they start thinking about props. The only consideration that diameter has as a criteria for selection is what gearcase do I have and what diameter prop is it designed for. Lets take your 75 Yamaha as the test mule. Say you want a prop. The logical process is to look at an application chart that identifies the diameter range for your engine. So lets do that. I went to the iBoats store, selected "Boat Propellers" and then keyed in Yamaha, 75 HP, and 2003. It then asks for two stroke or four stroke. Selecting two stroke turns up a long list of props from which I selected 15P which is shown below . From that list you can see the props for this engine fall into the 13 to 13-3/4 inch diameter range. Now go to the list for 13P and then go to the 17P listings. You will notice that as pitch goes up diameter comes down a bit. Conversely when pitch goes down, diameter comes up a bit. It all has to do with propeller design. So people simply need to stop thinking about diameter. A 15 or 16 inch diameter prop for your engine would be very wrong because it would not likely fit. Likewise selecting an 11 inch diameter prop would be equally wrong. Note also that a four blade prop may be a tad smaller in diameter than a three blade prop even though they both have the same pitch. Rake, blade shape, blade area, blade thickness, material, barrel design, vented or not, cupped or not, are all propeller design characteristics. Diameter is the least important provided the prop you select is in the range for the engine. Finally, engine HP and lower unit gear ratio determines what diameter range and pitch ranges are suitable for a given engine. Without looking it up, I suspect the gear ratios are very different for the two engines. Engine rpm, gear ratio, pitch, hull type and load are the prime factors in determining performance from any prop.

15P props

http://boatpropellers.iboats.com/Ya...andard&**********=290122091&*******=697856811

13P Props

http://boatpropellers.iboats.com/Ya...andard&**********=290122091&*******=697856811

17P props

http://boatpropellers.iboats.com/Ya...andard&**********=290122091&*******=697856811

 

[steelespike]

Re: Teach me effects of prop selection

Will a larger diameter prop with the same pitch put more load on the motor?
I would think the larger blade would grab and push more water, even if it was of the same pitch.

I always trim all the way down during hard take offs.

Boats are pretty much identical. Smaller one is a tiller and the other has single side counsel.

Weight difference between the motors is only about 30-35lbs.

I just picked up the larger boat and it came with about 30 gallons of gas that was put in the tank last summer. I should have sucked it out but decided to just use it. I'm not putting long heavy loads on the motor till I get some fresh gas in it. When I get down wind of the motor while idling it smells rich

Couldn't find a pro angler 16 in 2004 only a pro sport.at 970 The side console controls add some weight and probably 30+ gallons of fuel could actually be 200 lbs difference. Battery(s) gear? gear placement?

The actual trim angle could be different even though both have been trimmed in all the way. As pitch goes up generally diameter goes down and viceversa.
Where is the 30+ gallons of gas located in the bigger boat?How many gallons are typically in the smaller boat?where located?