Delta 7.5mtr - Yanmar 6by/bravo3 gear ratio question

[BrevikMarine]

Hi everyone.

Just wondering if anyone with experience on a similar sized Rib could perhaps assist with a gear ratio choice.

We Are assisting a customer fit a Yanmar 6by3-260z into a Delta that he bought with a knackered Mercruiser V8 diesel and a bravo 3 drive.

The drive has a ratio of 1.5:1 as it is.
we have dropped the props down to 21" pitch and we still struggle to get it on the plane, and nowhere near rated RPM of 4000. Currently we max out at about 3500rpm.

our choices would be 1.65:1, 1.81:1 or 2:1.

Once we have a rough idea what would suit then we can look at how to adjust it.

If I am not mistaken, the 1.81:1 will need gear adjustment of both the upper and lower sections.

The other ratio's will be adjustment of top or bottom.

would 2:1 be too much of a jump?
Any thoughts, inputs or criticisms welcome.

Stevo

[beamishken]

Originally Posted by BrevikMarine

Hi everyone.



Just wondering if anyone with experience on a similar sized Rib could perhaps assist with a gear ratio choice.



We Are assisting a customer fit a Yanmar 6by3-260z into a Delta that he bought with a knackered Mercruiser V8 diesel and a bravo 3 drive.



The drive has a ratio of 1.5:1 as it is.

we have dropped the props down to 21" pitch and we still struggle to get it on the plane, and nowhere near rated RPM of 4000. Currently we max out at about 3500rpm.



our choices would be 1.65:1, 1.81:1 or 2:1.



Once we have a rough idea what would suit then we can look at how to adjust it.



If I am not mistaken, the 1.81:1 will need gear adjustment of both the upper and lower sections.



The other ratio's will be adjustment of top or bottom.



would 2:1 be too much of a jump?

Any thoughts, inputs or criticisms welcome.



Stevo

Are you sure you have 1.5:1? Its an unusual drive even for the previous v8 diesel which would have usually had 1.65 if it is then your easiest option is switch the lower unit & you can go to 1.65 or 2.0. You need to gain 500rpm which swapping from 1.5-1.65 won't give you & at 21" props you cant reduce pitch by much. Smallest you'll find will be 20p bearing in mind you've dropped hp & your engine revs much higher. 2.0 will probably get you where you need to be & you can adjust with the props to fine tune

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[Limecc]

Assuming presently 1.5:1, then to gain 500 rpm with the 21" prop you need 1.71:1 ratio.

So if you select the 1.81:1 option you would go back to a 22" pitch prop to give you 4031 rpm at 36 knots.

If you select 2:1 option then you'd need a 22.5" pitch prop which equates to 4000 rpm. Hope this helps.

[beamishken]

Originally Posted by Limecc

Assuming presently 1.5:1, then to gain 500 rpm with the 21" prop you need 1.71:1 ratio.

So if you select the 1.81:1 option you would go back to a 22" pitch prop to give you 4031 rpm at 36 knots.

If you select 2:1 option then you'd need a 22.5" pitch prop which equates to 4000 rpm. Hope this helps.

If only it was as simple as the calculations make out

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[BrevikMarine]

Cheers for the replies

That's a valid point about the current ratio. It's 1.5:1 stamped on the housing but could have been changed inside previously. Not sure if it's the original outdrive for the boat either.

My thoughts were to go with a 2:1 as it would just be a lower unit change as you mentioned but the customer was concerned it may have been a bit too much.

Thanks very much

[beamishken]

Originally Posted by BrevikMarine

Cheers for the replies

That's a valid point about the current ratio. It's 1.5:1 stamped on the housing but could have been changed inside previously. Not sure if it's the original outdrive for the boat either.

My thoughts were to go with a 2:1 as it would just be a lower unit change as you mentioned but the customer was concerned it may have been a bit too much.

Thanks very much

The only 100% ways to check are either strip the lower unit & count the teeth on the gears then reference the gear charts (or ask here) or carefully mark the input & output shafts and turn the input until the output has done 1 full turn & see where the input shaft has landed. Its fairly easy to see the difference between 1.5,1.65 1.81&2.0 ratios. You can get bravo 3 props up to 30" pitch so even if the ratio is a little low at 2.0 you'll be fine. I'd imagine you'd end up on 24 or 26" pitch props with that boat engine combo with 2.0 ratio.
The other thing to consider would be replacing with a bravo 2 lower unit props are a lot cheaper and the drives are more robust if its going to be doing a lot of work the bravo2 would be a good swap

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