convert inboards to I/Os

[steelman]

Hi all I am a new be here.
I just got a good deal on a 31ft tiara open twin inboard 454 crusaders!
Has any one ever converted an inboard like this to jack shafted I/Os like Volvo duo props?
I think the balance should be good not moving the motors just changing angle. I would expect better cruise speed and better mpg
I know I will have to rework my transom end deal with steering and long shift cables what is your two cents?
Is it worth the work?
Tanks for your help

[marshmat]

Could it be done? Yes, most likely.

Is it worth it? Quite possibly no.

You'd have to calculate for yourself, of course, what it would cost to make the conversion versus what (if anything) you'd gain. This would have to take into account, of course, the maintenance and eventual replacement of the sterndrives- they don't exactly have a good reputation for longevity in saltwater. The conversion itself will cost you the purchase price of two sterndrives, custom fabrication of jack shafts, modifying the engine mounts, probably some structural reinforcing to handle the sterndrive transom assemblies, new or modified steering controls, and the labour to put all of this together in a cramped space under the boat's cockpit sole. Weigh all these costs against the benefits, if any (if the current setup was fairly well engineered, the props are appropriate and the shaft angle not too large, I doubt that you'd see large gains in speed or fuel economy, although there's a chance that close-quarters handling might be a bit better).

[steelman]

Thanks Matt
I was hopping someone had tried something like this and had some input.
the other thing I should of stared with maybe is how much gph improvement is a duo prop over inboard? My thinking is a I/O is more efficient than inboard and an I/O duo prop is even better.
I am good with the structure and the mechanics just not so hot with the math.
Thanks
Brian

[steelman]

UPS MY BAD CANT MAKE IT GO AWAY! Thanks Matt
I was hopping someone had tried something like this and had some input.
the other thing I should of stared with maybe is how much gph improvement is a duo prop over inboard? My thinking is a I/O is more efficient than inboard and an I/O duo prop is even better.
I am good with the structure and the mechanics just not so hot with the math.
Thanks
Brian

[J3]

In salt water, with a boat left in the water, sterndrives seem to commonly be shot after 10 years. I love stern drives in fresh water, but I don't know about salt... lots of cleaning and painting needed to keep them from turning to junk. In salt I think I'd prefer the simplicity of inboards and spend the money on engine mods to those 454s to get a pinch more performance if desired. For the sterndrive mod, what are you considering - $20 to $25k?

[J3]

P.S. is there a thread anywhere talking about power loss with shaft angle?

Through a bravo drive I've read hp loss is between 15 and 30 in this 330 hp range increasing with hp applied. How does the shaft angle compare in lost thrust hp? For mpg, how about drag from the inboard shafts and struts vs. outdrives?

[Karl2]

3-4% power loss through the transmission (I/O or reverse gear). Efficiency loss due to shaft angle will vary with angle but single digit %.
Drag from IB running gear = significant, increases with boat speed.

With same hp engine connected overall efficiency gain I/O vs. IB; 25 - 30% in the 30 - 35 knot speed range.

Karl

[steelman]

Hy J3 good point. But I think the cruise which is what I am looking at might be better. 10 years is good with me I know about upkeep on them.
thanks
HY Karl2 thanks
I like what you say! I now cruise at 3000/3200=20/25mph
and want to bump that mph up to improve gph.I just found a very good deal? on a tiara 31 open with crusader 350hp 454.
I think I can ge two duo prop Volvoes cheep they both had BB from the factory so I am hoping { ua that's not good}. I would like to know for sure before I pull the plug if I will see any gain.
Can I ask how you came by this #25 to 30%?
And thanks to all for your input!

Originally Posted by Karl2

With same hp engine connected overall efficiency gain I/O vs. IB; 25 - 30% in the 30 - 35 knot speed range.Karl

I would be rather interested too, where this statement would be based on???

[Karl2]

Originally Posted by apex1

I would be rather interested too, where this statement would be based on???

35 years of experience.

A broad statement, I know. But, as far as the efficiency gain for the IO the statement is, if anything, conservative.

Karl

[CDK]

I can imagine why people convert from stern drives to inboards, not the other way around.
After 7 stern drives I am glad those troublesome years are over. Go visit a seaside marina and observe how many I/O's are completely or partially removed for repairs.

There may be some efficiency gain in a narrow bandwidth, but the overall fuel consumption with stern drives will be a painful surprise.

Originally Posted by Karl2

35 years of experience.

A broad statement, I know. But, as far as the efficiency gain for the IO the statement is, if anything, conservative.

Karl

No, sorry. My 35 years of experience in boatbuilding told me there is just a marginal gain if any. And in terms of overall efficiency (including maintenance) there is a severe disadvantage for I/O drives.
The same is valid btw. for maneuverability when we are talking twin setups!

So, not only was your statement too broad, it was wrong.

Regards
Richard

[steelman]

Karl2
Thanks I feel the same way on this! That is it seams to me every thing I have read over the years tells me I/O is better on gas!! and now even more so with duo prop. But I would like some more facts.
NOW
Apex1 I would like to ask you the same thing if you don't mined.
Ware do you get your facts? and I would like to ask not to bash any one for there input,thanks.
CDK Yes I/Os need mat. but let me ask you this how many inboards sit and rot
with $4.00 a gal. gas x 50 gph? And one more thing this boat I am getting lost power on test ride with old owner and be for we could set the anchor we hit bottom and bent prop and shaft in three ft of water what will it cost to hall the boat and replace prop and shaft? AND please go easy on brother boaters.
PS
ADN IF YOU MUST PIK ON SOME ON PIK ON MY SPEELING SO i CAN REALY TELL U WAR TO well that's not nice is it?

THANKS to all of you for your time and thoughts!
steelman

Originally Posted by steelman

Apex1 I would like to ask you the same thing if you don't mined.
Ware do you get your facts? and I would like to ask not to bash any one for there input,thanks.

THANKS to all of you for your time and thoughts!
steelman

Building and operating boats and ships, with outboards, inboards, Vee drives, Z drives, Jets, Diesel electric, steam propulsion and some sailing craft, since 1973 I had the luck to get a brief overview of the advantages and disadvantages of different systems in marine propulsion.

By no means am I the expert in this field, but I am able to distinguish sales drivel from daily experience on the water / market.
The latter tells me that a I/O setup has only a small advantage in useable power supplied (duoprop for example), but the lifetime cost is a big disadvantage over the inboard.

And where did I bash on anyone???

Regards
Richard

[Karl2]

Originally Posted by apex1

No, sorry. My 35 years of experience in boatbuilding told me there is just a marginal gain if any. And in terms of overall efficiency (including maintenance) there is a severe disadvantage for I/O drives.
The same is valid btw. for maneuverability when we are talking twin setups!

So, not only was your statement too broad, it was wrong.

Regards
Richard

I though my first post was pretty clear. I'm talking about propulsion efficiency, not "Maintenance" or "Maneuverability" efficiency (B.T.W, I have no clue how to even try to measure or quantify those). I apologize if I confused anybody.
However, personally I prefer a conventional IB for maneuverability but that’s just because I’m more familiar and comfortable with that. I have friends that prefer I/O and could probably maneuver a twin IO boat around a dock as well as I can manage a twin IB.

I also suspect that maintenance cost, in a high hour, saltwater application would be higher for an IO set up.

Back to propulsion efficiency. The definition of this is subject to opinion. Let me try to explain how I look at it.

Let’s take a fictitious planing vessel; 35’ LOA, 12’ BOA, 13º at the transom, weighing 20,000 lbs and with the lcg 13’ from the keel/transom intersection.

With a pair of 300 hp (@ 3,500 rpm) IB I can calculate out 31.5 knots WOT with 2.0:1 swinging a 19x27x4C. I get good prop efficiency and reasonable prop loading.

If I change nothing in the boat but replace the IB with a pair of twin prop IO (same 300 hp @ 3,500) I get 36.0 knots WOT.
In reality the IO boat will probably be a bit lighter and with the lcg further aft, the ability to trim the IO will compensate for the aft lcg and, I submit, give higher top speed. But, for the sake of this simple exercise I’ll leave the boat the same in the examples.

31.5 to 36.0 knots is a speed increase of ~15%. In my original post I stated “overall efficiency gain I/O vs. IB; 25 - 30% in the 30 - 35 knot speed range”.
What gives ? Contradiction or conflict here ?
Perhaps, this is how I look at efficiency in this context (My definition, my opinion, disagree all you want):

31.5 to 36.0 knots. In this vessel what IB hp would it take to get to 36 knots ? So, using the same boat data I add hp and manipulate prop to get to 36.0 knots.
At 400 hp and swinging a 21x29x4C I get my 36 knots. In reality it would take more hp, the 400 hp installation will weigh more than the 300 hp installation but, again, for the sake of this exercise I left the boat alone.

In order for the IB installation to match the top speed of the IO installation an additional 33% hp is needed. The IO is 33% more efficient.
This also is a good match to a rule of thumb I have used for 35 years on IB: In a planning hull and below 40 knots the best you can hope for is that for every % of hp you add you can get .5% back in increased speed (I added 33% hp and got 15% higher speed).

There are not many real life applications that I know of, were the same boat is offered with IO and IB but there are a few. One that I’m somewhat familiar with from a few years back is a Four Winns Express Cruiser. I believe the boat was called 348. Don’t know if they still build it and if they do it may be called something else. As I recall there was a number of power options available. Amongst them were twin 5.7 gas IO and twin 5.7 gas V-drive. Same hp, same rpm. The IO boat ran 39 knots WOT, the V-drive boat ran 31 knots WOT. I’m sure this V-drive installation may not have been the shining star of efficiency but the efficiency gain here is consistent with what I’m claiming.

Given my definition and without maintenance or maneuverability in the mix I’ll stick with my original statement and “...as far as the efficiency gain for the IO the statement is, if anything, conservative”.

Karl