The X Dimension: Mercruiser vs Volvo

[joliette]

I have a Mercruiser sterndrive installation manual so have been able to calculate and mark up the x dimension (crankshaft centreline position) on my Settimo Velo. I don't have a Volvo Penta manual so was wondering if there was likely to be any difference in the x dimension? I would think not, but I would be interested to know for sure as I am in the process of getting sterndrive package quotes in for both Mercruiser and Volvo Penta options.

If anyone could shed any light on this or point me in the direction of a free download for the installation of Volvo Penta V6 and V8 sterndrives, detailing the x dimension, that would be appreciated.

 

[spannerman]

Basically the Merc and Volvo engines are identical GM based blocks,just the external bits are different so the crankshaft center line should be the same.
However if you are buying a new sterndrive package and installing it yourself you get a paper pattern with the transom shield which you use to calculate the X position.

 

[MapisM]

I have a Mercruiser sterndrive installation manual so have been able to calculate and mark up the x dimension (crankshaft centreline position) on my Settimo Velo. I don't have a Volvo Penta manual so was wondering if there was likely to be any difference in the x dimension?

Dunno if I understood your question exactly.
The X dim actually is the distance between the engine shaft and the bottom of the hull, and in this respect spannermann is right, there's no difference between VP or Merc blocks.
But I guess that you mean the distance between the engine shaft and the prop shaft, (which of course depends only on the outdrive), right?
If so, I'm afraid I can't tell you exactly this measure for VP outdrives, but I'd swear to have read that both SX and DP have the same engine/prop shaft distance as the Merc Bravo, which is 22" or so.
Below is a tech drawing for Bravo if that might help.
Anyhow, 'fiuaskme, a couple of Bravo I with full Latham hydraulic steering would be just the ticket for a Settimo Velo.

 

[joliette]

I was wondering if the engine centreline to prop dimension on a Volvo sterndrive was any longer than it is on a Mercruiser sterndrive, thus allowing the x dimension (hull bottom to engine centreline) to move higher up the transom.

This is an issue for me as I'm converting the boat from v drive to sterndrive, and have worked out (based on the Mercruiser installation manual that I have a copy of) that I'll need to recess the stringers by about 3 inches to get the front engine mounts on the correct level.

This will involve quite a lot of work so I was wondering if
a) there was any significant difference with Volvo Penta (eventhough the advice here is that Mercruiser sterndrives are more reliable)
b) there was a workaround, perhaps with some form of alternative engine mount.

 

[MapisM]

have worked out (based on the Mercruiser installation manual that I have a copy of) that I'll need to recess the stringers by about 3 inches to get the front engine mounts on the correct level.

This will involve quite a lot of work so I was wondering if
a) there was any significant difference with Volvo Penta (eventhough the advice here is that Mercruiser sterndrives are more reliable)
b) there was a workaround, perhaps with some form of alternative engine mount.

Well, re. a) as I said I really don't think so, though I don't have any technical evidence comparable to the above Merc drawing. Maybe volvopaul or someone else might help?
Re. b) instead, the workaround might be easier than you think. When you mention the "correct level" that you calculated, I guess you started from the basic principle of having the cavitation plate aligned with the bottom of the hull, right?
Because if so, you might consider that on fast boats it's common practice to raise it, also significantly. In my single engine 27', the cavitation plate is 5" 1/2 above the hull bottom, with the prop shaft less than 3" below the hull.
This is actually considered a rather aggressive setup, and in fact the boat is a bit of a dog in jumping on the plane, not to mention when maneuvering, particularly in reverse. But it works superbly and very efficiently at any speed from 30 to 60kts - and up to more than 80, on the same hull with higher power!
Now, the Settimo Velo hull was designed well before the modern tricks of my boat, like steps, pad keel and notched transom. But it's definitely a great hull, capable of serious speed. And with twin screws, neither the acceleration nor the maneuverability should be a big issue, even with a somewhat raised X dim.
So, I'd bet that she could actually benefit from a 3" raise.
Besides, you could consider fitting the Mercury ITS on Bravo drives, instead of the Latham I previously mentioned, because aside from providing the hydraulic steering (which I'd strongly recommend on any Bravo driven fast boat) they move the prop further aftward, and this allows - all other thing being equal - a more aggressive X dim.

PS: Do you have any pics of your restoration?
I'm sure they would be good for a thread at least as interesting as the other "new boat" ones we've seen lately.

 

[Scarron]

Before you cut any holes in the transom you are very welcome to bring your tape measure and note book, there are a couple of Triana 25's here in Southampton [very similar Sonny Levi hulls] for you to have a look over. One has twin 4cyl Volvos with 290SP drives and the other has twin V6s with DP drives. should be a good starting point for your deliberations.

I still think however that you shouldn't rule out a single V8 outdrive in your installation, much simpler and more efficient and whatever your budget is you'll get a better big one for the price of two smaller ones - if you see what I mean. You will need at least 320 hp tho so either the most powerfull small block or a big block.

 

[joliette]

My Mercruiser installation manual gives the following maximum increase for the x dimension, for boats above 50 mph:

Alpha 2-1/2"
Bravo One and Two 3"
Bravo Three 1"

So, if I push the x dimension towards max it will help to eliminate the need to recess the stringers so deeply. However, increasing the x dimension is going to impact handing characteristics, and I'm looking for good all round performance rather than just going all out for top end speed. I've seen Levi's drawings for installation of twin V8s in the Settimo Velo and, as far as I can make out, they look to be set on the x dimension.

The Triana is an identical hull form to the Settimo Velo ... I think I need to leave the Island and pay Scarron a visit in Southampton, as suggested! The V6 application will be of particlualr interest as first choice is to install twin V6s. However, in the meantime, if any of you guys happen to find yourselves with a tape measure to hand then the x dimension - from engine centre line to bottom of hull (as measured down the outside face of the transom) is 357mm ....

I will post some pictures here sometime soon.

 

[MapisM]

I wouldn't take those Merc recommendations too strictly.
First of all, they're surely meant to stay on the safe side, as any builder recommendation.
Secondly, putting the B-I and the B-II in the same league is ridiculous - the B-II is built to spin larger props on heavy boats, where high speed and raised X dim are not not really relevant.
Thirdly, they're probably based on the standard water pick-up, which might suck air with an aggressive X dimensions. But that's not a problem with the LWP (low water pickup), which is an OEM Merc option available for the B-I, specifically meant to avoid this problem (the cooling water is taken from four holes on the top of the nose cone in front of the prop, well below the standard side holes).
Lastly, they don't take into account the type of installation of the outdrive. It makes a big difference whether it's mounted with the ITS (or any other extension box, which btw did not even exist at the time Levi designed the boat) or not. Same goes for notched transoms, which allow higher X dim by virtue of the modified geometry of prop alignment vs. the hull bottom.

Anyhow, the X dim is exactly 19" on my boat, so well above the 357mm you mention, and it's a stock installation which came with a full Merc warranty. She does have a LWP on the lower case, though.
Bottom line, I'd bet that with a 3" raise the handling wouldn't be significantly affected on your hull, particularly with twin engines, and assuming that you'll have normally spinning props - i.e. outwards.

Looking forward to seeing those pics!

 

[joliette]

My copy of Mercruiser's installation manual - for gas engines up to 5.7L - is dated 2001. It does mention the problem of the water supply becoming aerated as a result of raising the drive, but only offers the solution of taking the cooling water by a through hull. So I guess things have moved on since 2001 with the introduction of LWP and ITS.

It'll be interesting to find out how the Triana's set up.

Incidentally, sterndrive.com put forward the view that the Bravo is an expensive option unless you have a big block engine over 350hp, as it's heavier / larger and will drag more in the water. They suggest small block engines are better off with the Alpha One ....

There are certainly lots of views and possibilities about, but sometime soon I will need to make a decision, buy some kit and cut two holes in that transom!

 

[MapisM]

Actually I don't think the LWP is available for the Alpha drive, because it's very different from Bravo in many respects - one of which is the water pump inside the outdrive case.
Btw, if they suggested a transom pick-up for the Alpha, did they mention that you should install also a separate water pump?
Anyhow yes, the Alpha is lighter, smaller and cheaper. But it's not just a matter of power, it's rather the torque and the kind of usage which can kill outdrives. If you're considering a 350 (5.7) block, and a boat usage also in some rough waters (which means possible ins and outs of the water), I'd steer clear of Alphas, even if I'm aware that it's a standard Merc package.
Btw, years ago they used to sell the Alpha also with the 454 block, which at that time had an output similar (330 hp, IIRC) to the current 350 block, but they soon dismissed that package because it proved unreliable.
Otoh, if you're going for V6 blocks, Alpha is the obvious choce.

 

[springer]

If you look at the Volvo Penta units you have the ability to specify a drive extension on most drive options. This will allow you to retain prop position in the water but step up the position of the engine in the hull. I don't know if Merc offer this option.

Just a thought.

 

[joliette]

If you look at the Volvo Penta units you have the ability to specify a drive extension on most drive options. This will allow you to retain prop position in the water but step up the position of the engine in the hull. I don't know if Merc offer this option.

Just a thought.

That's interesting, I haven't come across that. Do you have a link to an illustration of that? I'd like to find out more about how that could work for me.

 

[AndieMac]

Btw, years ago they used to sell the Alpha also with the 454 block, which at that time had an output similar (330 hp, IIRC) to the current 350 block, but they soon dismissed that package because it proved unreliable.

Years ago they would only have had the Alpha 1. The Alpha 2 top box is a lot stronger than the earlier model.
The big block torque would be the damaging factor for the Alpha 1 surely?

 

[MapisM]

There isn't such thing as an Alpha 2, AM. But I see what you mean. I can't remember exactly if the 454/Alpha package was pre or post '90 (which as I recall was when they modified the Alpha one into what became the Alpha one gen II).
I'm not aware that the gen II is "a lot stronger", though - do tell?
Anyhow, as a matter of fact, they never mated any Alpha with big blocks after the 454 mistake, whilst they always offered the Bravo as an option with all the small block V8s (and standard on the 6.2, whose output is just a tad higher than the 5.7).
To my tastes, the 5.0 is the borderline engine, where depending on the application I could go for either an A or a B.
From the 5.7 (included) upward, give me a B no matter what, thank you.

 

[joliette]

Before you cut any holes in the transom you are very welcome to bring your tape measure and note book, there are a couple of Triana 25's here in Southampton [very similar Sonny Levi hulls] for you to have a look over. One has twin 4cyl Volvos with 290SP drives and the other has twin V6s with DP drives. should be a good starting point for your deliberations.QUOTE]

I found myself in the Yard this afternoon with an idle moment to hand after winterising my Yanmar. So, with tape measure to hand I decided to measure up the only 3 sterndrive boats I could find ashore; 2 Mercuiser and 1 Volvo Penta. With the legs and transoms in place it was an approximate measurement that I was able to take and all three seemed to be set close to Mercruiser's recommended x dimension. They were all quite conventional hulls with none of the innovations that MapisM mentioned. I'd be surprised if the Triana installations have drifted very far from x ...

 

[AndieMac]

There isn't such thing as an Alpha 2, AM. But I see what you mean. I can't remember exactly if the 454/Alpha package was pre or post '90 (which as I recall was when they modified the Alpha one into what became the Alpha one gen II).
I'm not aware that the gen II is "a lot stronger", though - do tell?
Anyhow, as a matter of fact, they never mated any Alpha with big blocks after the 454 mistake, whilst they always offered the Bravo as an option with all the small block V8s (and standard on the 6.2, whose output is just a tad higher than the 5.7).
To my tastes, the 5.0 is the borderline engine, where depending on the application I could go for either an A or a B.
From the 5.7 (included) upward, give me a B no matter what, thank you.

Sorry M, I meant A1 Gen 2, bit busy this time of year running in and out of the office

My techy informs me that the Gen 2 has a beefed up gear set and bearings in the upper box, noticably different to the Gen1. In fact he says it could take 350 - 400 hp driven from a small block.
He did say though you wouldn't do this from a big block, he used the words small block 'pony power' and big block 'draft horse' power, which I'm sure you knew anyway

As I said previously the big Bravo 2 was no use to me as the leg hangs too low on the transom for trailer/ground clearance, and won't raise high enough to dry on hard sand, besides the GBP2,500 extra cost to buy.

 

[MapisM]

My techy informs me that the Gen 2 has a beefed up gear set and bearings in the upper box, noticably different to the Gen1. In fact he says it could take 350 - 400 hp driven from a small block.

I take your word for the stronger upper in G2 alphas, but I still would prefer bravos for anything above 300hp or so, though I agree that also the higher torque of a big block can make a difference.
In fact, think about it: Merc mates the 377 - which is still a small block with "only" 320hp - only with bravos, whilst from a marketing viewpoint it would be in their interest to offer the engine also with the lower price tag of the alpha.