Reducing prop pitch for heavier loads?

Simon Guest · 09 October 2014, 09:50

Hi all,

Have been running an air floor Zodiac Classic MkIIC with 15hp Mariner 2 stroke for a few years. It was great when the 3 kids were pretty small - all 5 of us planed comfortably. Now the kids are a lot bigger - all up load with 5 is probably 370kg, compared with 290kg when we started. Now we can't plane with all on board. (But when 3 wait on the beach, knee-boarding behind still works a treat.)

So here's my question: are we likely to be able to get planing again by dropping to a 7 inch pitch prop? (Current prop is Mercury Black Max 3 blade 9 inch diameter, 8 inch pitch). I know I'll lose top end speed, I'm choosing not to care about that. I haven't really got a feel for how much difference an inch will make ...

All suggestions and speculation welcome. (I know, I need a bigger boat/engine - not an option for now.)

cheers,
Simon

dave3235 · 09 October 2014, 10:20

[QUOTE=Simon Guest;650094]Hi all,

Have been running an air floor Zodiac Classic MkIIC with 15hp Mariner 2 stroke for a few years. It was great when the 3 kids were pretty small - all 5 of us planed comfortably. Now the kids are a lot bigger - all up load with 5 is probably 370kg, compared with 290kg when we started. Now we can't plane with all on board. (But when 3 wait on the beach, knee-boarding behind still works a treat.)

So here's my question: are we likely to be able to get planing again by dropping to a 7 inch pitch prop? (Current prop is Mercury Black Max 3 blade 9 inch diameter, 8 inch pitch). I know I'll lose top end speed, I'm choosing not to care about that. I haven't really got a feel for how much difference an inch will make ...

All suggestions and speculation welcome. (I know, I need a bigger boat/engine - not an option for now.)

cheers,
You could try it but 1 pitch lower which will raise the revs by 2 to 300 rpm's.You are probably at the max weight wise.You could get a 7p and send it to steel developments and they could take it to a 6p which would be more noticeable in raising the revs hope this helps.

Fenlander · 09 October 2014, 11:02

Your Classic is probably rated to 35hp so for a full load you are under engined. You are also already down a pitch on the standard 9 prop fitted from new. In the same circumstance (Honwave alloy floor 3.5m with Mariner 15) I just accepted the non-plane when fully loaded to avoid the expense of a second prop and possible over-rev when normally loaded.

69cmw · 09 October 2014, 11:59

35HP when fitted with floor boards.
25HP when fitted with an air floor
At least that's what mine said on the spec plate on the transom.

Max... · 09 October 2014, 14:04

Quote:

Originally Posted by Simon Guest

...Now the kids are a lot bigger - all up load with 5 is probably 370kg........are we likely to be able to get planing again by dropping to a 7 inch pitch prop?

In a nutshell, no chance! That is a hell of a load for a little 15 to cope with - either lose the load or increase the power. No other practical option.

paddlers · 09 October 2014, 15:22

I'm afraid the above advice seems good. My Honwave 3.5 Ally floor planes easily with 5 of us with the 20hp Tohatsu (max on transom plate) four stroke but won't get out with my Mercury 15 2 stroke with stainless prop.

I can't believe the extra 5 horses make so much difference!

Max... · 09 October 2014, 15:55

Quote:

Originally Posted by paddlers

I can't believe the extra 5 horses make so much difference!

It's a more than 30% increase in power, not just 5 horses. Like going from a 150 hp saloon to the souped up 195hp one.

paddlers · 09 October 2014, 16:00

Quote:

Originally Posted by Max...

It's a more than 30% increase in power, not just 5 horses. Like going from a 150 hp saloon to the souped up 195hp one.

And you know it straight away. I always imagined the 2 stroke element might have narrowed the gap!

Peter_C · 09 October 2014, 17:55

I dropped my prop pitch from 17" down to 15" since the boat is most often loaded heavy. It definitely helped. I also have a Stingray lifting wing on the motor that keeps the bow down and allows the boat to plane at around 12 MPH, with 6 people onboard (No dive gear).

Simon Guest · 11 October 2014, 03:13

Thanks for all the input. Maybe I should look at slightly larger engines, then. I do recall, though, standing in the London Boat Show about 10 years ago, walking around carrying a 15hp was just fine, but a 25hp I could hardly lift. My Mariner 15hp 2 stroke weighs 35kg. The Mariner 25hp 2 stroke weighs 48kg. Tohatsu 25hp is similar. Tohatsu 18hp only 41kg, but I can hardly justify buying a new engine for another 3hp.

Ah well, Summer's coming (for us Southern Hemisphere folk). Happy days ...

cheers,
Simon