propellers, pitch and material ?

[simonfraser]

ok, need a new one, tohatsu 3.3 2 stroke 2013 for my 7m 1T trimaran

the plastic 6 pitch one which i have is as cheap as chips

aluminium is available in 6 and 7 pitch

max boat speed using the OB is 5 knots, before i spun the prop in the gravel, using the plastice 6 pitch one
hull speed is way more, so am i only getting 5 knots due to the prop or due to the 3.3 HP ?

alu is way more money, what to buy ?

 

[simonfraser]

bump, anyone ?

 

[PuffTheMagicDragon]

I don't think that, not knowing what kind of boat you have in mind anyone can give a meaningful answer to your question. I am inclined to think that you are talking about a tender; perhaps I am wrong. In any case, since the 6-inch plastic prop "is as cheap as chips" I should think that is the way to go. With a 2-metre GRP clinker style dinghy that weighs just over 50kilos I find that an ancient Yamaha 2b is more than enough to push it at 5 knots with the throttle at less than half.
A new prop is cheaper to try than a new engine, IMHO.

 

[Topcat47]

I would guess you're more likely to be under-engined. I know a 7m trimaran is fairly easily driven but I'm surprised to see a Tohatsu 3.3 used as an auxiliary. If a new 6" pitch prop gives you 5 knots, a 7" should drive at just under 6kts if the engine isn't underpowered for the vessel.

 

[seumask]

In my light weight tender, with one person on board I was able to get the tender to plane with the standard plastic prop on the same engine, I'm guessing it did around 10-12 knots, so in my view its the power of the motor that's the issue here, the max speed was governed by air getting to the prop starting cavitation.