prop size

[kimtre]

can any help me what size prop should i have on my boat it is a 55ft trad with a 1500 bmc and a tmp box this is a 1 to 1 ratio the prop that is on the boat is a 16x10 left hand twist how do you do the sums for this. many thanks lucy lockit 42

[WotEver]

can any help me what size prop should i have on my boat it is a 55ft trad with a 1500 bmc and a tmp box this is a 1 to 1 ratio the prop that is on the boat is a 16x10 left hand twist how do you do the sums for this. many thanks lucy lockit 42

Midland Chandlers will do this for you (they did for me) for nowt. As I recall, you give them the boat draft, boat length and engine size and they'll tell you the ideal prop size.

 

Tony

[blodger]

can any help me what size prop should i have on my boat it is a 55ft trad with a 1500 bmc and a tmp box this is a 1 to 1 ratio the prop that is on the boat is a 16x10 left hand twist how do you do the sums for this. many thanks lucy lockit 42

Have you problems with the one fitted?

It's usually best to have as big as will fit between skeg and uxter leaving recommended clearance (2"?)

Draft comes into it since bigger props need to be submerged somewhat.

Pitch is adjusted for matching engine/gearbox torque and rpm of shaft for optimum loading

Bit of an expert job predicting best prop - Crowthers

Since prop is already on improving on it is about ascertaining whether you are overpropped or underpropped where it matters at low/idle revs or at max (not reaching full revs) and going for a greater or lesser pitch one accordingly if size is OK.

Someone with a bit of knowledge needs to witness prop & engine performance really.

Formulas can be misleading but guide first choice.

 

I increased size of my prop slightly to 17" and increased the pitch so that it is now slightly overpropped without stalling on idle and without cutting max revs (that I do not often use) by much. Different engine & gearbox to yours.

 

I have had a small springer and BMC1500 when I changed the prop on that because the orignal shed a blade (taking what was on offer cause I was on holiday) it ran unbelievably better. However, I would caution not to expect to much by tinkering a bit!

Nothing wrong though with following a hunch and changing the prop keeping the orignal for a spare or if the change is adverse IMO

[paulcatchpole]

Midland Chandlers will do this for you (they did for me) for nowt. As I recall, you give them the boat draft, boat length and engine size and they'll tell you the ideal prop size.

 

Tony

 

There's a downloadable propcalc program for Windows into whcih you put much the same info, and it gives you the calc.

 

Can't remember where I found it, but it'll be out there somewhere...

 

PC

 

Edit: It's from Castle Marine

 

Putting some figures in for your boat, very estimated:

 

 

Gives:

 

 

So a small 13x10 but efficient (55%bar) giving you 8mph flat out.

 

PC

[alan_fincher]

can any help me what size prop should i have on my boat it is a 55ft trad with a 1500 bmc and a tmp box this is a 1 to 1 ratio the prop that is on the boat is a 16x10 left hand twist how do you do the sums for this. many thanks lucy lockit 42

I don't know TMP boxes, but are you sure about there not being any reduction.

 

Normally this type of marinisation of a BMC would have some reduction, (we have 2:1 on the similar 1800 engine).

 

I'm kind of surprised that a 1500 will drive a 16" prop with direct drive, albeit one with a small pitch.