Which prop should I use?

[madsurfe]

Lately in a couple of threads there has been talk about using another prop for performance or replacement.

This link from Mercury Marine presents a detailed explanation of how it all works, certainly worth a read. Read all four chapters.

How propellers work

[breamN2wheels]

hey john, havnt had time to read that mercury section, but mim took me out on the boat today and told me about all the exelant fish you guys get into. you guys make a great a team good luck james.

[madsurfe]

As to the speed winning you the extra dollars....nah, I reckon it's your skill as an angler that does the job.

ref: AngelfishSuzuki 60 four stroke

the quote above is from another thread but important because the way I see it even a good angler appreciates having more time to fish by getting to Spot 'X' earlier. I also enjoy the fun factor ie: I'm a revhead.

My experience with motors, pacticularly 2 strokes came from riding road registered MotoX bikes in Enduros & Road Trials (1980's). I also did the occasional MotoX. Competing successfully in Enduros meant reliability and thus we would only perform a modification if the performance gain did not impact reliability.
I also waterskied and Jet Skied during the off season (Summer).
Most of the water ski boats were powered by 140hp outboards.
The 'ol adage of making better use of what you already have ie: improve your skill before getting better tools is something that I always consider.

I'll continue below

[madsurfe]

So Mim and I have the little 'ol Muddleduck which is a Savage Snipe with a Yamaha 30cv. It had a great holeshot, there are many non-boaters that can attest to the holeshot ability. It would over rev at 22-23 knots with the std prop. The motor setup does not have power trim/tilt so all changes to leg height and trim, bar moving the trim pin have to be done off the water. I had already maximised the power to weight ratio and wanted more so the search was on to reprop.

I went to Just Propellors at Osborne Park (WA) and purchased an Aluminium replacement. Std Yam prop is 9 7/8" X 12" so I bought a 10 1/8" X 13" after discussing the options with Mick. I raced homed, fitted the prop and in my hurry ran over a Breambuster with the boat trailer, cest la vie. Yes I drove off forgeting that I had the rod leaning against the car oops!

The performance was better than the std prop yet was disappointing. I had achieved little,

1: I had lost my great holeshot
2: speed increase was minimal
3: cavitation was still a problem in turns (no improvement from Std)

So back home and drop the motor a little. All this did was reduce some of the cavitation, initially, but it actually got worse when I went real low. I had to accept that with a dinghy hull and 'T' keel there would always be aerated water hitting the prop.

I returned to Just Propellors and discussed my results with Mick. We agreed that there is not much of a choice for props in this hp range. I went the extra distance and purchased the SS prop. It is similar in measurement to the Std Yamaha prop. So I fitted the Solas Y B10" X 12" and went for a test off Hillarys with Mim. We filled the livewells and started mucking around with trim.

Bloody Nora! Crikey Dick! (just for you Brody) and all that, geez we were scooting around all right and I was a little edgy cos' I wasn't used to doing 25 knots plus over 1 foot chop.

I told Mick that I was happy with the prop and spent the next few outings getting the leg height and trim setup. I tested all settings by the rise/fall in speed with my GPS. The gain was in a prop that was designed better and made from a better material.
It's dimensions are similar to the std Yam but it doesn't flex like an Alu prop. The best part of an inch is given away in pitch with an alu prop, Mick knows the calculation. It has better grip when scooting over chop and in moderate speed turns. Sharp high speed turns are not really an option in the dinghy and weren't possible with the std prop.

In all honesty the good result comes from having power to burn. Our boat had the best holeshot out of any Breaming boat up to 60hp, well in WA. I was lucky that the prop design suited what i already had in place.

As Angelfish said in the other thread:

Size, pitch, cup and rake are far more significant in this circumstance

In summary I balanced the boat well when building it and made use of a prop that lifted the bow a little more. I have trimmed the boat to provide the best control in 2 foot of chop. This cost me a knot on flat water but the hull design is a compromise so I have done the best I can with what I have and it's bloody good fun to drive. I also understand the boats limits and will not endanger myself or others by trying to drive it flat out through heavy weather.