Top speed, prop size and pitch?

[bjornaa]

Hello everyone!

maby the wrong place for my post in this big forum, but its a SIB so i put it here

A know some about prop sizes and pitch, but when i try the different props out on the boat I don’t get the expected result.

The boat, Zodiac futura mk 2, 40 hp mercury 2t, on a normal day loaded a bit of gear and a two person crew.

we got the boat with a 10-1/2" x p13 prop top speed WOT (5500 rpm) 43 km h or 27 MPH which is ok according to the manual that states 23-29 MPH with that prop.

That was great the first week but we soon wanted more speed, and it did a lot of blowouts or spinouts... it seemed like the engine had much more power.

so we went with a 10 x p16 supposed to get us going at 30-36 MPH acording to the manual, the day came with much excitement, but what a great fail... WOT (5500 rpm) no problem... but it was slightly slower and with more prop spinouts the same 43 km h or 27 MPH was recorded!

speed was recorded on GPS bough times... i don’t understand that the 10 x p16 doesn’t give us the speed we want... the 10xp16 looks a lot smaller than the 10-1/2xp13, maby just the pitch doing that,,,

so we are back at looking at props, with 2 new unwanted on our hands.
im lost and don’t know what to get, is it the boat? Its well inflated and engine run great! i have heard ppl. saying they go a lot faster with same boat and engine!

im looking at a these atm.
http://propulse.jetshop.se/ListProducts.aspx?cat=110

I found a used 13 x p17 that will fit... i know its big but will it work? im thinking no spinout and the pitch must be high enough to make the engine struggle to reach its rated rpm ie. high speed, but maby not healty for the engine..

I hope someone has an input thx!

[Sharkbyte]

Spend some time nailing your optimum trim angle, engine height, and weight distribution (can make a huge difference) and you should see some improvements.

That's not to suggest that your current prop, or the one you are eyeing up, are the right ones.....but best to get the basics right before throwing silly money at an underlying problem.

I would also expect more than 27 mph from your setup.

Cheers

Wayne

[kelson]

I have a zodiac futura mk2 with a Nissan 40 hp (3 cylinder two stroke) and I hit 29 mph at 5500 rpm (wot) with a light load. Depending on your load 27 mph may be a reasonable top end. Remember, this is a soft bottom boat. Water resistance increases exponentially as your speed increases. It is going to take a lot more than a prop change to get higher speeds with your boat. First off, make sure that your boat is pumped up fully. If the pressure drops as the tubes cool off in the water you definitely want to top them off. Moving weight to improve your trim can help a little. I run with trim tabs and they help the boat to get onto a plane at lower speeds, but don't make much difference in top speed. I have a propulse prop. It is really nice to be able to adjust the prop pitch for varying loads, but I didn't get a significant top end bump. The four bladed propulse prop did improve my hole shot. My Mk2 futura with the 40 on the back is just a little slow for ideal single ski riding. We ride wakeboards and hydrofoils behind it. Both of those toys work well at lower speeds and are much easier to start on. I also have a low pole installed on my zodiac to make water starts a lot easier. I don't think you would get much more speed even with a 50. If you want a much higher top end you probably need a rib. The solid hulls are much more efficient at high speeds.

[bjornaa]

Thx a lot guys!

Sharkbyte:
i will look at the setup next time i get the chance, ill read up on guidelines!

kelson:
i know your setup, i heve been reading many or your old posts real nice stuff, the tripod is great was it homemade?

i do not have the manual on the futura mark II, so i dont know how much air pressur it needs in the different tubes, and i need a pressure gauge!

Ribs are expensive over here, and not easy for beach launch... but i can see that they could be better for many reasons top speed beeing one of them..

Anyone ellse has some speed recordings for this boat?

[kelson]

Your Futura should be inflated to 240 mb (which is 3.4 psi). As the temperature of the tubes fluctuates the pressure changes significantly. The recommended range is 220 mb (3.1 psi) to 270 mb (3.85 psi). Get a pressure guage and keep the tubes and speed tubes at the proper pressure. It makes a big difference!

The tripod is a standard adjustable ski pole. I bought mine from West Marine.

The boat is definitely fast enough for most watersports. The wakeboard is fun, but there is no wake! The hydrofoil can jump a lot higher than I want go. We do have to go early or late to avoid other boats. Even at 22mph the ride in the boat is harsh if crossing wakes.

[jyasaki]

Originally Posted by bjornaa

we got the boat with a 10-1/2" x p13 prop top speed WOT (5500 rpm) 43 km h or 27 MPH which is ok according to the manual that states 23-29 MPH with that prop.

so we went with a 10 x p16 supposed to get us going at 30-36 MPH acording to the manual, the day came with much excitement, but what a great fail... WOT (5500 rpm) no problem... but it was slightly slower and with more prop spinouts the same 43 km h or 27 MPH was recorded!

3" of pitch change and no change in WOT rpm? Should have given you 400-600 rpm less when you lengthened the pitch (though you'd also get a couple of hundred back from the drop in diameter.)

Something doesn't sound right. Are you sitting on a rev limiter or something?

jky

[bjornaa]

I know it dosent seem right! i hope to try the 13"xp17 it should get the rpm down... not that i whant it down i just whant a result of some cind...

[C-NUMB]

its not always easy....my bombard reaches 22-23 knots with original 11 pitch prop and 2 persons+some gear. Tested today with a slightly bigger cheap Turning Point prop, 13 pitch. Same speed...it does not make sence. Maybe the slight increase in diameter is too much.

Top speed so far has been 26 knots, single, light load with the 11 pitch prop in good conditions. Second person decrease the speed pretty much.

The air floor moves plenty, think its time to buy the Bravo superturbo, manully just cant get it hard enough.

[Sharkbyte]

I'm not sure whether you guys find the same, but a nice chop on the water usually gives me at least another couple of knots.

Flat calms feel like riding on a lake of treacle, and are pretty much the only sea state to induce the odd episode of sudden, and extreme, prop cavitation.

Thankfully, truly flat calms are a rarity here. ;-)

[jyasaki]

The speed thing I can see. Most boats take a certain amoount of power to reach a certain speed, then pumping a bunch more in power in yields a very small increase in speed.

The rpm question is a little more straightforward (as hydrodynamics go): an increase in pitch equals greater load, which should knock the rpms down. Increase in diameter (with the same pitch) equals more load, which should knock the rpms down. There are, of course, other variables that may come into play: blade shape, prop weight, etc, but they shouldn't affect things all that much.

So the question is, why isn't that happening in this case?

jky