Yamaha F70 & Buster Lx

Pry

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Jun 25, 2014
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I have a 2011 Yamaha F70 mounted on an aluminum hull Buster Lx and was wondering what would be the best prop to get the most top speed out of the combination.

I currently have an aluminum Yamaha 13 1/4 x 17 K prop.
I get 6300 rpm and 32.7 kts / 37.6 mph WOT fully trimmed up when I'm the only person in the boat. The motor is currently mounted as high as possible and the anti-ventilation plate is just above the water.

The engine has recommended WOT range of 5300-6300 rpm. The gear ratio is 2.33:1.

The boat:
buster-lx-speed-test.jpg

Length:
5.04m / 16.5 ft
Beam:
1.98m / 6.5 ft
Weight:
415kg / 915 lbs
Hull V-angle:
16 degrees
Number of persons:
6
Power range:
40-60hp
Performance:
lx_teho_en.jpg


The boat is basically made out of a Buster L by moving the driver and passenger forward and giving them individual seats. The Buster L:
Buster_L_jetty.jpg


Since the boat is a bit nose heavy I have to trim it as high as possible to get the bow up. This increases the speed but I'm not sure if the current prop is the optimum choice getting the bow up and running this close to the surface at this angle. I believe the prop slips quite a bit when I trim it all the way up. It rises quite a fountain behind the boat:


What do you guys think what would be the best prop to get the most out of the combination?
 
Last edited:

steelespike

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Apr 26, 2002
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19,069
Your numbers are pretty good 37.6 produces about 13% slip pretty normal. I've seen less on here but not often and some times suspecting some of the input.
I would say that a 70 that produces almost 40 on a deep V boat with a 1,000 lb dry weight is doing well.
Some one may be along with some suggestions.
 

Pry

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The 16 degrees isn't considered very deep V angle over here in Finland.
When I move to the back seat the boat gains speed which indicates that the bow doesn't lift high enough. For a comparison the Buster L (750 lbs) which uses the same hull but has more weight distributed to the back gets 36 mph with two people on board powered by a 50 hp Yamaha and an SS prop.
 
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Pry

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Yes it has power trim. I was wondering if there was a stainless prop that would give me more bow lift than the 17" aluminum and still keep the revs up. I'd quess the 13 1/2 x 17 Yamaha pro series would drop the rpm too much even though I would have less resistance by lifting the bow higher. How would the 13 1/4 x 16 or 13 1/4 x 14 performance 3 props compare to the current 13 1/4 x 17 aluminum prop? How would the 13 x 17, 13 1/2 x 16 and 13 1/2 x 14 painted ss props differ from the performance 3 props?
 
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steelespike

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Apr 26, 2002
Messages
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When selecting a prop diameter is a secondary consideration.You may note that as pitch goes up diameter tends to go down.
Pitch is the primary consideration.Rule of thumb a 1" pitch change will add or subtract
about 200 rpm. If you look at a page of props for your motor you will note a quite vast range of diameters.
When considering a 17" prop for your boat a 17" should behave like a 17". Design features will add bow lift,stern lift ,
hole shot,grip,ability to stay on plane at reduced throttle.In searching for the best top speed you may lose some hole shot,
boat may become fussy trying to stay on plane,may become sensitive to added weight.
I'm sure someone will be along with some suggestions for bow lift etc.
 

Pry

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Jun 25, 2014
Messages
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Thanks for the advice! I've also heard a larger diameter causes some drag and lowers the rpm a little.
Does someone have an idea what would happen for the rpm if I installed a 13 1/2 x 17 Michigan Wheel Ballistic prop? The prop seems to give a better bow lift and good speed but I'm not sure if the rpm would drop too much. Also not sure if a 13 5/8 x 15 Ballistic would give me too much rpm.
I've heard a rule of thumb is to go 1" smaller for a SS prop but not sure how would that rule work on this particular prop.
 

steelespike

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Joined
Apr 26, 2002
Messages
19,069
If the prop gives more bow lift it should temper any loss of rpm by the diameter change.
1/4" would not be more than 50 or 100 rpm if anything.
A 15" prop decrese will add About 300 to 400 rpm.1/8 diameter means nothing.
With a good setup like yours reducing pitch will increase rpm and lose speed
Again as pitch goes down diameter may go up.
 
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