Propeller query

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Dmc

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I have just done cutlass bearings on my boat and noticed the props are different sizes ie.17.7x12 and 17.5 x10.the boat came with 2 new props which are these sizes as well.could these be right for this boat.any ideas much appreciated!
 
It sounds as if you have two pairs of props. It's a bit like two pairs of socks - you're wearing a black one and a brown one, and therefore have an identical pair in your drawer.
 
2" pitch difference is significant (ignore the minor diameter difference) and will put very different loads on the engine(s). Twin engine boats would normally have matched props otherwise the engines will run at different rpm. Either size could be right for your boat but I suggest you go to a propeller specialist and ask them to first check the pitch of the props (may be different from what is stamped o them) and do the sums for your boat to determine which size is better.
 
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Dmc said:
I have just done cutlass bearings on my boat and noticed the props are different sizes ie.17.7x12 and 17.5 x10.the boat came with 2 new props which are these sizes as well.could these be right for this boat.any ideas much appreciated!
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does it go OK?
Were you aware of an issue at all?
 
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Tranona said:
2" pitch difference is significant (ignore the minor diameter difference) and will put very different loads on the engine(s). Twin engine boats would normally have matched props otherwise the engines will run at different rpm. Either size could be right for your boat but I suggest you go to a propeller specialist and ask them to first check the pitch of the props (may be different from what is stamped o them) and do the sums for your boat to determine which size is better.
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Sorry diameters are both 17.5
 
Wing Mark said:
does it go OK?
Were you aware of an issue at all?
Click to expand...
No we did 7 hour journey in it and it ran really well only found out when we lifted it out.we have an old surveyors report and he mentions the sizes and condition of them and even comments on spare pair again noting sizes but doesn't say they are wrong.was hoping there was reason they were like that
 
Are the gearbox ratios the same? Sometimes props are different to compensate for different gearbox ratios and hence prop rpm's and thrust, although two inches does seem a lot.
 
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I accompanied my brother when going to look at a motor cruiser that was capable of being used both at sea and on inland waterways. The seller said that he used a coarse pitched pair of props at sea, while he fitted the finer pitched pair for canal and river use, so that he could motor at low speed as is required in these waters, but at a higher rate of revs that was better for the engine. I suspect that you have one of each pair fitted and should swap out one of them, depending on where you intend to use the boat.
 
VicS said:
Are they handed, ie left and right, if so maybe the LH and RH gear boxes have different ratios ....???
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Agreed. Twin gearboxes and props should counter rotate - ie. same specs but one LH turning and one RH turning.

Can your props be matched in that way or could that be the reason for previous fitment (two 12" turning one way and two 10" turning the other so forced to mix pitches to keep them counter rotating?)
 
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Boat may have had replacement gearbox at some time in its life, for reasons of budget or availability the drive ratio may be different from original box.
Simple check.
Go at WOT on each engine separately , if they achieve roughly the same revs, they are probably OK.
If vastly different , really depends as to how much faff and expence you want to go to to sort it.
A decent local prop shop can tweak the prop blades but at around £100- £150 .
Guess from prop pitch its a slow displacement boat.
 
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As others has said above, check the transmission ratio's. It's more common to have different ratio's for each direction. My money is on this being the issue.
 
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ChromeDome said:
Agreed. Twin gearboxes and props should counter rotate - ie. same specs but one LH turning and one RH turning.

Can your props be matched in that way or could that be the reason for previous fitment (two 12" turning one way and two 10" turning the other so forced to mix pitches to keep them counter rotating?)
Click to expand...
Moodysailor said:
As others has said above, check the transmission ratio's. It's more common to have different ratio's for each direction. My money is on this being the issue.
Click to expand...
 
Yes the props are handed and are fitted to perkins 4108s.the boat is out for winter so hope to get it sorted while it's out.it seemed to run nicely.is it possible these props could be correct
 
Dmc said:
Yes the props are handed and are fitted to perkins 4108s.the boat is out for winter so hope to get it sorted while it's out.it seemed to run nicely.is it possible these props could be correct
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Not really. If same gear ratios a 10" and a 12" will put notably difference in load on the two engines - especially if trying to keep them art same rpm.
Rule of thumb is that 1" changes rpm by 200 so to get equal thrust you'd need to go 400 rpm more on the one fitted with 10"

I'd suggest to look at the four props you have and pair them. If 2 x 12" or 2 x 10" is right can be calculated, but you'd need test runs to make sure they fit your need.

I don't think you mentioned what boat make and model. Others might have something comparable and could share insight
 
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ChromeDome said:
Not really. If same gear ratios a 10" and a 12" will put notably difference in load on the two engines - especially if trying to keep them art same rpm.
Rule of thumb is that 1" changes rpm by 200 so to get equal thrust you'd need to go 400 rpm more on the one fitted with 10"

I'd suggest to look at the four props you have and pair them. If 2 x 12" or 2 x 10" is right can be calculated, but you'd need test runs to make sure they fit your need.

I don't think you mentioned what boat make and model. Others might have something comparable and could share insight
Click to expand...
 
ChromeDome said:
Not really. If same gear ratios a 10" and a 12" will put notably difference in load on the two engines - especially if trying to keep them art same rpm.
Rule of thumb is that 1" changes rpm by 200 so to get equal thrust you'd need to go 400 rpm more on the one fitted with 10"

I'd suggest to look at the four props you have and pair them. If 2 x 12" or 2 x 10" is right can be calculated, but you'd need test runs to make sure they fit your need.

I don't think you mentioned what boat make and model. Others might have something comparable and could share insight
Click to expand...

Different pitch props will be correct if the gear box ratios are different.
If he pairs them he will find the two 12" props will be the same hand and the two 10" one will be the opposite hand if different ratios is the reason for the different pitches
 
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