Hydradrive help

vdubr928 · 12 July 2016, 04:05

Hi, I'm in the US and I have a 92 7.4 Hydradrive. I am not having too much trouble finding parts but a manual and advice are really hard to come by. I pulled the drive at the end of last season and cleaned the filter while resealing everything I took apart and it has worked great but is now having trouble shifting after high speed runs. I am looking for someone that understands these or can turn me on to a used one that is in good condition. I appreciate any responses and assistance.

Thank you,
Carl

beamishken · 12 July 2016, 07:32

You can replace with mercruiser alpha or bravo as they use the same spline and transom drilling pattern
Need to use the yamaha inner transom shield bolted to the mercruiser outer

Dirk Diggler · 12 July 2016, 09:57

I've found this company helpful ..... Boats.net: Outboard Motors, OEM Marine Parts, Boats for Sale and a good source for parts

vdubr928 · 12 July 2016, 16:21

Quote:

Originally Posted by beamishken

You can replace with mercruiser alpha or bravo as they use the same spline and transom drilling pattern
Need to use the yamaha inner transom shield bolted to the mercruiser outer

Beamishken;
That is great news on my end. I have found many posts on US forums saying that is not true. Have you done one this way? What else is required? How do you manage the shift linkage? Also, the Yamaha has an external fresh water pump. Do I just bypass that and use the Merc pump? Does it connect the same way? I'm sorry to have so many questions but I would like to have everything ready and do the swap quickly so I can continue my season.

Quote:

Originally Posted by Dirk Diggler

I've found this company helpful ..... Boats.net: Outboard Motors, OEM Marine Parts, Boats for Sale and a good source for parts

I have used them for many of the parts I purchased last year. They are reliable and have about half of the parts available. It has not been difficult to find parts but I'm skeptical of tearing into this thing without a manual. If I can easily swap a Bravo on there, I won't bother rebuilding this one. It will be easier to sell in a few years when I'm ready to upgrade with a merc on it.

Dirk Diggler · 12 July 2016, 16:49

Quote:

Originally Posted by vdubr928

Beamishken;
That is great news on my end. I have found many posts on US forums saying that is not true. Have you done one this way? What else is required? How do you manage the shift linkage? Also, the Yamaha has an external fresh water pump. Do I just bypass that and use the Merc pump? Does it connect the same way? I'm sorry to have so many questions but I would like to have everything ready and do the swap quickly so I can continue my season.

You can't fit a Bravo Drive to a Yamaha Transom. I suspect the suggestion was to replace your current drive and transom with a Merc one, using the Yamaha inner transom plate to hook it all up.

You'd also need a Merc raw water pump and a shift bracket to make it all work.

vdubr928 · 12 July 2016, 17:47

Quote:

Originally Posted by Dirk Diggler

You can't fit a Bravo Drive to a Yamaha Transom. I suspect the suggestion was to replace your current drive and transom with a Merc one, using the Yamaha inner transom plate to hook it all up.

You'd also need a Merc raw water pump and a shift bracket to make it all work.

What I mean to say is that in the US most people say the hole isn't the same and the inner transom won't work. I'm wondering if anyone has done a writeup on this. I have heard enough people say it will work that I am seriously considering buying a bravo (and transom assembly) but it would be nice to have some confirmation from someone who has done this. I have suspected that much of the advice I've seen posted in the US forums is from mechanics who really just want you to repower. They also say parts are not available and I have had no trouble finding parts.

I have a couple of more questions.
1: Doesn't the mercruiser have a raw water pump in the lower drive? Do I just remove the impeller and run an external raw water pump?

2: How do I know what gear ratio to purchase? I do not know the current ratio or how to find it but I know mine is a 300hp 7.4 and was turning a 16x16.

yam man · 12 July 2016, 20:32

Are you running the hydra drive on a petrol ? The ratio on the petrol version are 1.50:1

beamishken · 12 July 2016, 21:49

You will need drive and shield the drilling pattern in the transom is the same. if you use a bravo drive there is no pump so you need an engine mounted pump.if you use an alpha it has a built in pump.
Bravo is stronger drive but more expensive alpha is said to be good for 450hp petrol but needs carefull use of throttle with the big blocks but will be cheaper. for gearshift you just use the merc shift plate with merc adapter kit to your 33c cables from the helm

beamishken · 12 July 2016, 22:01

Mercruiser drives come in a variety of ratios 1.5:1 being prety common so easy to swap like for like ratio wise but it wouldn't be hard to run a prop calculation through mercruisers website for your boat and engine combo the engine will be the same as a merc engine of the same era so prety simple to do prop calcs for your boat with the merc drive
You could also consider a bravo 3 but would need to reduce the ratio to 2.2 or 2.0

yam man · 13 July 2016, 05:34

Or simple enough get 7.4 bell housing of merc same block . The only thing maybe a problem is the trim sender as May mess up the Yems an retard the ignition .

beamishken · 13 July 2016, 09:37

Bell housing suggestion is a good idea as plenty mercruiser bits about you could then swap to merc steering ram too.
I'd imagine if the trim sensor needs to be in the system then you could leave it plugged in somewhere in the engine bay to give the system a static reading which should work
If you want the trim gauge it's no big deal to wire in a merc resistance gauge to the dash
That's assuming the merc trim sender is different resistance but easy to check with a meter

jwalker · 13 July 2016, 12:36

Quote:

Originally Posted by vdubr928

.....
....I do not know the current ratio or how to find it but I know mine is a 300hp 7.4 and was turning a 16x16.

Turn the drive input shaft with the drive in gear until the prop turns one full turn. Keep track of how far the input shaft turns and this will give you your ratio. EG: Input shaft 1 1/2 (1.5) turns - prop (output shaft) 1 turn will equal 1.5:1

If the drive is in gear, simply turn the engine instead of the input shaft. Be sure the engine can't start!!

Dirk Diggler · 13 July 2016, 13:02

Quote:

Originally Posted by beamishken

alpha is said to be good for 450hp petrol

beamishken · 13 July 2016, 23:04

Quote:

Originally Posted by jwalker

Turn the drive input shaft with the drive in gear until the prop turns one full turn. Keep track of how far the input shaft turns and this will give you your ratio. EG: Input shaft 1 1/2 (1.5) turns - prop (output shaft) 1 turn will equal 1.5:1

If the drive is in gear, simply turn the engine instead of the input shaft. Be sure the engine can't start!!

Won't work on a hydradrive they need to be spinning to get the oil pressure to engage the clutches

vdubr928 · 14 July 2016, 16:32

You guys have made my year. I was very worried when I started having trouble. Nobody in the US has accurate info about this. I bet there are hundreds of these things in scrap because people don't know this is possible. I'm going to start collecting parts. I want to go with a Bravo 2 so I can probably make the stock Yamaha raw water pump (mounted on engine) work. I can still easily get the impellers for it.

Yam Man, I curious about your comment on the trim sender. My Yamaha has the Delco EST ignition. I don't know if that is the same as yems? Can you explain a little? I am a low voltage guy so I may be able to work around this or just trash the est and go with a mechanical advance.

vdubr928 · 14 July 2016, 16:33

Quote:

Originally Posted by jwalker

Turn the drive input shaft with the drive in gear until the prop turns one full turn. Keep track of how far the input shaft turns and this will give you your ratio. EG: Input shaft 1 1/2 (1.5) turns - prop (output shaft) 1 turn will equal 1.5:1

If the drive is in gear, simply turn the engine instead of the input shaft. Be sure the engine can't start!!

Being a Hydrostatic drive, this is impossible. It sounds like I have a 1.50:1 setup since I have a petrol engine.

Edit, This was already mentioned above. Not sure why i didn't see all the posts when I came in this morning,

beamishken · 14 July 2016, 16:58

If your going bravo two you will need to down pitch to around 2.0:1ratio due to the large diameter prop on the bravo
Your already on a fine pitch prop so I'm guessing it's a big heavy boat?

vdubr928 · 14 July 2016, 17:28

Quote:

Originally Posted by beamishken

If your going bravo two you will need to down pitch to around 2.0:1ratio due to the large diameter prop on the bravo
Your already on a fine pitch prop so I'm guessing it's a big heavy boat?

So should I just stick with a Bravo 1? I'm sure it will handle 300hp.

It is a big cruiser. It had a 16x16 on it. I put the 13 on there to help it get out of the water. I suspect a bearing issue in the drive is generating the extra heat that is causing this issue. It may be that the 16x16 was fine and the low WOT RPM was caused by drag inside the drive due to whatever bearing is failing.

I have read it is advantageous to run a larger diameter prop on a cruiser but I am not very knowledgeable about this. I'm a car guy with pretty good mechanic skills but this boat stuff is new to me.

beamishken · 14 July 2016, 17:40

Bravo 2 or 3 will be best for a big cruiser due to the bigger blade area but in reality any of the bravo range will do they all have there plus and minus points
Mercury have a good prop calculator which will give you the best ratio and prop size for your set up
I think personally I'd go for 2.1 bravo 2 but props are expensive for them and much cheaper for bravo 1 and more readily available

vdubr928 · 14 July 2016, 18:17

Quote:

Originally Posted by beamishken

Bravo 2 or 3 will be best for a big cruiser due to the bigger blade area but in reality any of the bravo range will do they all have there plus and minus points
Mercury have a good prop calculator which will give you the best ratio and prop size for your set up
I think personally I'd go for 2.1 bravo 2 but props are expensive for them and much cheaper for bravo 1 and more readily available

Mercruisers Prop Calculator doesn't go back past 2000. That will be a much higher HP version of my engine. I would't mind the expense of the bravo 2 props if I can get it right the first time. :-)