Top speed, prop size and pitch?

bjornaa · 12 July 2010, 16:09

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 · 12 July 2010, 22:57

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 · 13 July 2010, 14:39

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 · 13 July 2010, 15:52

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 · 13 July 2010, 17:10

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 · 14 July 2010, 14:25

Quote:

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 · 14 July 2010, 17:01

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 · 14 July 2010, 22:58

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 · 15 July 2010, 00:35

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 · 15 July 2010, 14:18

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

9D280 · 15 July 2010, 14:49

Quote:

Originally Posted by jyasaki

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

Quote:

Originally Posted by bjornaa

we got the boat with a 10-1/2" x p13 prop
<snip>
so we went with a 10 x p16

At a guess, the 3" pitch change is being nicely offset by the half inch diameter change.

bjornaa · 15 July 2010, 15:27

9d280

What you are saying makes sence to me, but everywhere I look all the online prop calculaters they only use the pitch to finde the speed (rpm ect.) and I guess the prop slip % that apparently it huge on a sib...

What I'm thinking is:

the more whater is forced back the bigger the forward speed, big prop big pitch must be the way to go...

azzurro · 15 July 2010, 20:26

Not quite like that. Prop pitch and diameter is just like bicycles.

You will not be able to rev the engine up to its optimal level, thus your engine will not render as much power.
If you use too short pitch and tiny diameter the rev limitator will stop the engine (and if it doesn't, the engine will burn).

Thus, to get as much power as you can from your engine:
high load -> short pitch short diameter
one man -> big pitch long diameter

jyasaki · 16 July 2010, 16:54

Quote:

Originally Posted by 9D280

At a guess, the 3" pitch change is being nicely offset by the half inch diameter change.

Seems a bit out of balance though.

My recollection of the rules of thumb is 200-250 rpm per inch of pitch, and 100 rpm per 1/4" of diameter.

so +3 and -3 should still change by about 300 rpm. Then again, there may be other stuff involved too... Or I may be remembering wrong.

jky

bjornaa · 17 July 2010, 11:06

Thx a lot guys!

I will get back to this post whith updated after i try something new, have a great summer!

nice.diver · 18 July 2010, 16:39

bigger boat, my MKIII with johnson 40, scuba gear, or just people

22-24mph

with both of my props
11 3/4 X 17
11 3/4 X 15

bjornaa · 18 July 2010, 18:34

Well thats strange, one would think the mkIII was faster, and with your big props and high pitch... its wird that it does not go faster?

the props you have :

11 3/4 X 17
11 3/4 X 15

How did you diside them? engine manual, or what... maby they are too big and then you dont get up to the rated rpm? therfore the low speed...

kelson · 18 July 2010, 23:57

My propulse four bladed prop in a MK2 Futura with a 40 hp Nissan 3 cylinder is 10.4" in diameter and adjust from 12 - 16" in pitch. Like I said before a sib has a very inefficient hull at higher speeds. Resistance or drag rises at a much higher rate than a hard hulled planing surface. A few more hp, a different prop and changes in load are going to have more effect at the low speed end. As you get up in speeds the small changes are going to have less impact on top speed. My hole shot has gotten better, but my top speed hasn't varied a huge amount. Yes, the original prop on my outboard did not hit the target wot rpm. I did change props to get the correct wot rpm.

9D280 · 19 July 2010, 16:10

Quote:

Originally Posted by jyasaki

Seems a bit out of balance though.

My recollection of the rules of thumb is 200-250 rpm per inch of pitch, and 100 rpm per 1/4" of diameter.

so +3 and -3 should still change by about 300 rpm. Then again, there may be other stuff involved too... Or I may be remembering wrong.

jky

I agree with the pitch ballpark, as I have proved it umpteen times with differnet boats but the diameter thing is anything but linear....... (My old Yam 55 on the current boat I uppped diameter by 1/4" and went from chickening out as it passed 6500 still accelerating (old school engine with "big bang" rev limiter

) to topping out nicely at 5500, both props standard ally 3 blade Yam at 14" pitch.

C-NUMB · 19 July 2010, 16:42

Have also noted that If using different brands of propellers, the results are not always logical, there is so much variation in the design between various brands.