Gear ratio prm 160

[Kevin davis]

Hi all I have a prm 160 gearbox.i just fitted new engine and gearbox but was told as I had gone from 1to1gearbox for a 2to1setup I would have to change prop as a 2to1 would rotate in opposite direction.i have been informed today that new prop isn't needed as you can simply change direction of pull on gear cable.sounds like an easy fix to me but am concerned that gear ratio for forward would be different for astern and may damage box in the long run.any advice appreciated

 

[Tranona]

A PRM 160 is available in 2:1 or 3:1 reduction and ratio is the same in both directions. prm-newage.com/media/File/160D%20man.pdf There is no problem with running continuously in either direction - that is one of the selling points of this type of gearbox. However if your propeller was for a 1:1 it will not be suitable for 2:1 - the shaft speed is far too high so the propeller will be too small diameter and too large pitch. You may find a 2:1 gearbox not suitable for your boat because you may not be able to fit a large enough diameter propeller.

Changing engines except for direct replacement is often more complicated than it seems. What boat is it, what was the old engine and box and what is the new? Presumably your "new" engine is secondhand as the 160 went out of production in 1997 replaced by the 280.

More information will help give better advice.

 

[Kevin davis]

Hi thanks for your reply.the boats a versatility 25ft I took a seazed 3.8 BMC out and replaced with anouther slightly newer 3,8 BMC.not sure what make old 1to1 box is I have been told I need a 22x16 prop but was hoping I would get away with old prop knowing gearbox can run both directions

 

[PCUK]

Direction has nothing to do with prop' size. Your old prop' will be running at half the speed it did with the old 1:1 box so at maximum engine revs you will get roughly half the boat speed you did previously. A prop' supplier will be able to estimate the correct size of prop' required. Presumably that is where the 22 x 16 you have been quoted has come from.

 

[Kevin davis]

Direction has nothing to do with prop' size. Your old prop' will be running at half the speed it did with the old 1:1 box so at maximum engine revs you will get roughly half the boat speed you did previously. A prop' supplier will be able to estimate the correct size of prop' required. Presumably that is where the 22 x 16 you have been quoted has come from.

That's right I got prop info from castle marine.i gave them all boat details and they did the conversion.looks like I need new prop either way.thanks for advice much appreciated

 

[Tranona]

22*16 sounds much too big. Problem is that the 3.8 BMC was offered in a range of outputs from 50-80hp and from different marinisers - and all far too powerful for your boat. The different outputs are achieved primarily by governing maximum revs - so a 65 would rev higher than 50. So to stand a chance of getting a decent prop recommendation that is big enough to absorb the power but not so big that it restricts revs you need to know the output and maximum revs of the engine you have.

With regard to the impact of change in reduction ratio and therefore shaft speed I have done some calculations based on 65hp at 2600rpm (which is approximately right, but I don't have the actual figures) and with a 2:1 reduction a prop of 20*14 is suggested and with direct 1:1 a 13*7. Looking at photos of the boat there is plenty of room for a 20" diameter prop. What diameter is the existing propeller?

Before you spend any money on a new prop find out the key figures of rated HP and RPM. I based my calculations on displacement of 3 tonnes and waterline length of 22'

Hope this helps

edit see you have a recommendation. Go with that as if it is wrong it is their responsibility. TBH you have so much excess power you will never need to explore high revs!

 

[bristoljim]

Hi all I have a prm 160 gearbox.i just fitted new engine and gearbox but was told as I had gone from 1to1gearbox for a 2to1setup I would have to change prop as a 2to1 would rotate in opposite direction.i have been informed today that new prop isn't needed as you can simply change direction of pull on gear cable.sounds like an easy fix to me but am concerned that gear ratio for forward would be different for astern and may damage box in the long run.any advice appreciated

 

[bristoljim]

what size is the shaft ??

 

[Kevin davis]

Hi m8 the shaft size is 28.5mm or1.122 inch

 

[Kevin davis]

what size is the shaft ??

28.6mm or 1.22 inch is the shaft size mate

 

[Tranona]

That is 1 1/8" Imperial. Bit on the skinny side if you have one of the higher rated engines, but should not be a problem as you will never use all the power.

 

[bristoljim]

Hi m8 the shaft size is 28.5mm or1.122 inch

A few more questions - what are your expectations for the use of the boat ? - what is the budget for a prop - as a 22" prop is going to be in thhe order of £500 & much to big for a 1.125" shaft. Where are you based ? .I may be able to help with a slightly smaller prop at an afordable- price if you PM your email to me. As Tranona says you do not need all the available power.

 

[Kevin davis]

A few more questions - what are your expectations for the use of the boat ? - what is the budget for a prop - as a 22" prop is going to be in thhe order of £500 & much to big for a 1.125" shaft. Where are you based ? .I may be able to help with a slightly smaller prop at an afordable- price if you PM your email to me. As Tranona says you do not need all the available power.

Hi m8 I'm based not to far from you in Newport my email is kevedwarddavis@gmail .com.the reason I've changed boxes is because I bought replacement engine thinking I was going like for like but bought slightly more modern unit mistakingly.which doesn't match up with my old 1to1 box so went with 2to1 that came with replacement engine.jusy trying to make it work now.she just going to be used for rod fishing for pleasure not expecting to be towing ships lol

 

[bristoljim]

is email kevedwarddavis @gmail.co ?

 

[Kevin davis]

is email kevedwarddavis @gmail.co ?

Yes mate but no gaps you appear to have gap between@

 

[Kevin davis]

Yes mate but no gaps kevedwarddavis@gmail.com you appear to have gap between s and@