Repower Inboard Prop Pocket Hull with Outboards?

[StandAlone]

Hello all,

I'm new to this forum but not new to boats. I've had everything from 15' to 42' and the current boat we have is a 1988 Sea Ray 390 Express. It's original configuration is twin Gas 454 Straight Inboards with Prop Pockets.

We plan to repower her in the next 12 months and are weighing options. Obviously the easiest solution is a new set of drop in engines. However, since we've decided to keep this boat into retirement as I get older I would really rather maintain outboards than engines down in the bilge if I am to continue to do my own maintenance.

Most would say sell the boat and buy one that is set up with outboards but since the closest to what we'd want I've seen is $1 Million bucks plus that's not going to happen.

I'm really interested in repowering this boat with twin outboards on a bracket. I realize the transom will need to be reinforced and knees added for strength but my biggest curiosity is how the prop pockets will affect the handling and performance if they are left as is with the outboards mounted directly behind them using the tops of the pockets as the reference for engine height.

The boat would still be used as a cruiser with desired top speed around 26 knots.

Looking forward to opinions or hearing from someone that has done this type of repower.

Thanks, Ken

 

[Angélique]

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Hi Ken, welcome to the forum . .

I would think to have the 2 outboards in 2 wells just in front of the transom would be a better idea then to have them on a one or two brackets(s) aft of the transom.

The wells could be where the prop pockets are now, the outboard props would be on the same place that way . . :idea:

Many sailboats are repowered that way, I'll post one for info soon, hang on . .

For general info ---> Google: - And also try Google Images:

outboard well - - | - - outboard motor well

outboard well plans - - | - - outboard motor well plans

outboard well construction - - | - - outboard motor well construction

outboard well design - - | - - outboard motor well design​

Good luck !

 

[Mr Efficiency]

Maintaining large modern outboards yourself could be a problem, if you want to stay under warranty, what with the computer programs and electronics that are needed to satisfy the service requirements.

 

[ondarvr]

Interesting project, but will two outboards do it?

 

[Mr Efficiency]

The wells could be where the prop pocket are now, the outboard props would be on the same place that way . . :idea:

!

Problem possibly would be getting the full turning arc without fouling the tunnels with the outboard leg structure, 35* each way I think.

 

[Mr Efficiency]

Interesting project, but will two outboards do it?

You certainly can get them big enough nowadays But I'd like to know more about the current cruise speed setting, if it is less than 25 knots, I'd say the outboards might not be the way to go.

 

[ondarvr]

HP is available, but prop selection may limit the performance, two props half the size of the current ones.

 

[Angélique]

See also the (small) outboard well thread ---> Wooden dory/skiff​

 

[Angélique]

. . . . Many sailboats are repowered that way, I'll post one for info soon, hang on . .

Atom Voyages ---> 2012 Blog​

[ . . . . ]​

1970 Alberg 30 Project

[ . . . . ]​

Some of the projects we did together were replacing the rigging and chain plates and adding an outboard well to replace the missing inboard gas engine. Unused thru-hulls were removed and glassed over, deadlight windows replaced, and seemingly dozens of other jobs large and small were completed.

^ Non-tilting enclosed outboard well. It does fully swivel for side thrust.​

Last fall we launched the boat and are continuing the work at her slip here in Brunswick. The owners come down on weekends and we sometimes take time off from the ongoing jobs to do some sail training in local waters. The 6hp Tohatsu Sailpro in the outboard well in the lazarette locker is a fairly minimal size, but does manage to push the boat along about 5 knots in calm water.

1968 Alberg 30 Update

As mentioned in the 2011 blog, we have a customer's 1968 Alberg 30 among our backyard boat fleet. This boat has been an ongoing project since we found her neglected at the beginning stages of a previous owner's major renovation at a yard in south Florida. Work continues and this year we have finished most of the exterior work including adding all new exterior teak and redesigning the companionway to incorporate stainless steel tracks and security bars with hinged doors as well as Lexan drop boards. A Norvane windvane was added for self-steering and new furling genoa and battenless mainsail have been ordered.

After seeing the outboard well on the later model A30 mentioned above, the owner of this boat decided to repower with the same 6hp Tohatsu SailPro. Yes, that size motor is considered under-powered for this size boat. The idea is to use actual sailing skills for passage-making and use the motor only as limited propulsion in calm waters. This time I designed and built a tilt-up well that allows the motor to stay in place permanently and easily be tilted up out of the way for drag-free sailing while still being contained within a watertight locker.

^ Enclosed tilt-up outboard well.​

^ Motor down in motoring position. You can see the prop on this 25" shaft is almost as low as the original inboard prop was.

^ Motor swung up in sailing position.​

[ . . . . ]​

Triton #503 Project Completion

[ . . . . ]​

^ The Triton Catalyst on her cross-country journey.​

^ Two Atom Solar-trackers installed. The outboard is tilted up for storage. Motor can swivel for side thrust which is useful in this packed marina.

---> Outboard Motor Well Demonstration​

Triton 503 - Catalyst: Outboard Motor Well Demonstration

Outboard Motor Well Demonstration​

Demonstration of the tilt-up outboard motor well we constructed on the Pearson Triton, Catalyst. This is a four-stroke 6hp motor with 20" shaft and high-thrust prop.

 

[Mr Efficiency]

It will lose quite a bit of weight with the outboards, but if it is a heavy and fairly slow boat as it stands, I don't like the idea of outboards that much, for the reason you mention, props, those big banger outboards are built for speed, not plugging along. Gearing and prop diameter says 30 knots plus is their go.

 

[Angélique]

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Hi Ken,

Study High Thrust Outboards. - And also listen to the other opinions and suggestions here, they could be better than mine . . :idea:

Here's some High Thrus Outboard Info, in a for this thread's application a bit edited repost of...

A Houseboat that Sails ? ---> post #15​

There are at least 10 points you need to take care of to make your outboard work nicely on a particular boat:

- 1. - The chosen engine needs to produce the right amount of power.
- 2. - The chosen engine needs to produce the right amount of torque.
- 3. - The chosen transmision needs to have the right gear ratio for the boat + engine + prop combination.
- 4. - The chosen prop needs to have the right diameter.
- 5. - The chosen prop needs to have the right pitch.
- 6. - The chosen prop needs to have the right number of blades.
- 7. - The chosen prop needs to have the correct rotation direction in relation to the engine.
- 8. - For doubles choose a left and a right turning version to eliminate prop walk.
- 9. - Have the right leg length.
10. - Nothing is right if it ain't all together right. - So each point has to be right in relation to all other points.​

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A High Thrust outboard version vs. a Standard outboard version has:

- In general¹ exact the same engine with exact the same specs, so the engine has exact the same power and torque output at exactly the same revs at the crankshaft.
- A higher gear ratio, this means lower revs and a higher torque available at the prop shaft.²
- Often the ability to house a larger prop diameter.³
- Often a more durable transmission.⁴
- Often a more durable leg.⁴
- Often a bit more weight.⁵
- With the right prop more thrust at low boat speeds⁶ compared to the Standard outboard version with also the right prop.

¹ Meaning: for most brands with High Thrust versions of outboards in their program.

² More torque delivery at the prop shaft means a higher force load on all components in the transmission and leg, especially on the bevel gears in the end.
- Therefore manufacturers often choose to use the next size up leg for High Thrust versions.

³ If the manufacturer has chosen to use the next size up leg for the High Thrust version then there's room for the next size up prop diameter.
- If the manufacturer choose to use the standard leg with only a different gearing for the High Thrust version then there's only room for the standard prop diameter, and then often a 4 blade prop instead of a 3 blade is used for the High Thrust version, as often just more pitch on the small diameter 3 blade prop would be less effective for this application.

⁴ If the manufacturer has chosen to use the next size up leg for the High Thrust version then the force on all components in the transmission and leg will be lower than on its original design application. Hence less wear and thus more durability.
- If the manufacturer choose to use the standard leg with only a different gearing for the High Thrust version then the force on all components in the transmission and leg will be higher than on its original design application. Hence more wear and thus less durability. Plus the higher force increases the chance that some components will break even before they're worn.

⁵ If the manufacturer has chosen to use the next size up leg for the High Thrust version then this will add some weight.
- If the manufacturer choose to use the standard leg with only a different gearing for the High Thrust version then the weight will be almost the same as with the Standard outboard version.

⁶ With a low speed of the boat the prop grips better on the water when it has a low speed itself. To realize this the prop needs to produce more water displacement per rev. And to do so the prop needs lower revs and a higher torque at the prop shaft.<sup>7

7</sup> Note: the power at the prop shaft remains the same as with the Standard outboard version. This is only about the High Thrust version's more effective way to transfer the power to the water at low boat speeds.

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How to know if a High Thrust outboard has the next size up leg or the standard leg with only a different gearing:

First indication from the specs is weight:
- If the High Thrust version has more weight than the Standard version then this is an indication for the next size up leg.
- If the High Thrust version has almost the same weight as the Standard version then this is an indication for a standard size leg with only a different gearing.

Indication from the ability to house a larger prop diameter:
- If the High Thrust version has the ability to house a larger prop diameter than the Standard version then this is an indication for the next size up leg.
- If the High Thrust version only is able to house the same prop diameter as the Standard version then this is an indication for a standard size leg with only a different gearing.

Indication from the availability of props with a larger prop diameter as for the Standard version:
- If there are for the High Thrust version props available with a larger diameter than what fits in the Standard outboard version then this is an indication for the next size up leg.
- If there are for the High Thrust version only props available with the same diameter as for the Standard outboard version then this is an indication for a standard size leg with only a different gearing.

Visual indication from pictures and/or videos and/or seeing it IRL in the flesh:
- If the High Thrust outboard's leg looks and measures a little beefed up compared to the Standard version's leg, and/or there are other differences, then these are indications for the next size up leg.
- If the High Thrust outboard's leg looks and measures exactly the same as the Standard version's leg then this is an indication for a standard size leg with only a different gearing.

And just ask:
- Ask the dealer and/or the importer and/or the manufacturer etc. etc.. - And seek confirmation for the info they give . . :idea:

Good Luck !

 

[Mr Efficiency]

You won't find "high thrust" 250 hp + outboards, angelique, the propellor diameters available don't cater to it, These babies are built for speed, heavy, slow boats (admittedly both terms are a bit subjective) are not a good fit. Maybe one day they will have an offering that suits , but they don't seem to exist at the moment as far as I can see.

 

[Angélique]

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I didn't check for the availability of High Thrust 250⁺ HP outboards, my thought was if they're available it could be an option here.

If they don't exist then the idea is out.

It would need a hell of a strong outboard well though . .

And also attached to the transom . . :idea:

 

[Mr Efficiency]

You are a mine of useful info, Angelique, and an asset to this site !

 

[PAR]

Welcome to the forum.

Nope, there aren't 350+ HP high thrust options in outboards, which is what this boat requires, along with it's 300+ gallon fuel tankage. She's built relatively light and originally used pretty low grade hardware (replace all the plastic thru hulls). This permitted them to get up and scoot, so twin 375 Cats, or 454 (Mark IV's) were very common. They don't like to be driven hard into a small swell or current. Unfortunately the tunnels cause her to squat badly while coming up and it seems like forever for them to do so (needs tabs). They also have low speed handling issues, worse than usual, even for a full plane hull form. Even with the 750 HP Cats, they're a little under powered, but they can manage 30 MPH in smooth water, but once it chops up, they beat you up pretty good.

To directly address Ken's questions and concerns, the simple answer is outboards just aren't a good propulsion option for this boat, for several reasons. First is the radical weight distribution changes this will cause. You could trim it out by moving stuff, but this makes the project more involved. Tankage will need to move, stringer issues will likely crop up other then the obvious reinforcements necessary for a bracket mount. Layout reconfiguration might be a welcome thing to think about, as the gen set and other stuff can now be moved to where they will not get soaked in the bilge anymore, but (again) more renovation and redesign stuff. Then you'll have to justify the costs. A set of crate Mark IV's are in the $3,000 range (long block, ready to install). This is pretty easy to swallow, compaired to just the cost of a bracket and a set of used, but good Black Max's, let alone new ones and reconfiguring the boat for this setup.

These boats had a number of issues. If it was me, I'd focus on making her a much better version of what she is, with some new or reman engines. All the hardware on these was low end stuff (have you fixed all the port and windshield leaks?). Production habits also was on the low end of the spectrum, so have the stringers and other structural elements inspected and repaired. A lot of the 1980's versions used really cheap covering materials which tended to delaminate in time. It's all bolt on stuff, but replace this sort of thing would make a better boat. A boat of this era will have some need for tabbing reinforcement and other repairs and this is where I'd toss my money, after a repowering. For the cost of one new 350 HP outboard, you could make this puppy really nice and solid.

In short, I'm not trying to insult you or your boat Ken, but these were what we think of as entry level craft (for their size and performance envelop). Because of the way (and era in which) they were built, there's lot of improvements that can (should) be made. You can make her solid and reliable or spend your budget, on a bracket and outboards, with all the other stuff on the back burner.