Repower a powercat

[Pieman]

I have a fountaine pajot Maryland with 125 hp Yanmar nearly 5000 hrs and am thinking to repower Rather than new diesels I am looking at 200 to250 hp outboards. Reasons as follows. Half the cost Quilter much easier servicing and less weight. Hopefully higher cruising speed am now limited to about 8 knots Any thoughts opinions advice appreciated. And yes I realise that we will use considerably more fuel thanks for any help as it’s an important decision

[Sailmonkey]

Is there any hull structure that will support outboards? You’re talking about doubling the horsepower and attaching engines that the hull structure was never designed for.

[kapnd]

Replacing inboard diesels with outboards requires some careful engineering.
Diesels are torquey, and produce power at relatively low rpms, making them ideal for pushing the large diameter props that perform so well at sub planing speeds.
Outboards are primarily designed to produce high speeds at high revs, on light planing hulls.
The available gearing and prop aperture of the outboard severely limits prop choices, so you may not be able to properly match this power to your boats needs.
I’ve seen outboard conversions that were practically useless for low speed maneuvering, and had quite a lot of trouble getting the boat to plane, but performed well once over the hump.
Be careful!

[Pieman]

Thanks for the replies. Some of this model boat have much larger diesels fitted We have to fit pods to the transom to take the outboards Prop selection is going to be interesting but I will research this as thoroughly as I can

[Searles]

Hull integrity to take that much h p plus that much weight right aft ,consider pitching motion loads first thing consult the designer,be guided by his advise

[Sailmonkey]

Originally Posted by Pieman

Thanks for the replies. Some of this model boat have much larger diesels fitted We have to fit pods to the transom to take the outboards Prop selection is going to be interesting but I will research this as thoroughly as I can

But what are the pods going to attach to? Larger diesels are mounted to the stringers in the hull, specifically placed there to transmit the thrust to the hull. I’d bet there is no structure in the transom that is capable of transmitting thrust to the hulls without some serious work.

Here’s a pod being added to a boat that was already configured for stern mounted propulsion.

https://m.youtube.com/watch?v=kO2n3spEnWk

Very different than what you’re proposing.

[mcarthur]

There have been some powercats that moved from inboards to outboards, so it certainly is possible. I assume you aren't meaning diesel outboards?
You'll need to stop cavitation during hobbyhorsing, so almost certainly a long shaft will be needed.
Again, assuming not diesel outboards, you'll need to change your fuel store to petrol (aka "gas"), which has very different storage and management needs.
Another change is to your rudders - will you need them still? They won't work as well unless the outboard is directly in front (very unlikely you'll accomplish that). Will the outboards be turnable? If so, you'll be adding that control (hydraulics likely) and changing your steering.

[Ifitsworthdoing]

I agree that outboards are a repowering solution well worth looking at, bearing in mind the design and engineering challenges mentioned above.

Have you looked at the new diesel outboards from OXE? Their 2 litre engine series are available in 125 to 200hp (based on a horizontally placed marinised Opel/Vauxhall/GM car engine) and while rather heavy and expensive have significant advantages over petrol outboards: (1) you can stick with the much safer fuel type and have no need to convert the main fuel tank to petrol which can be tricky in itself; (2) considerably lower fuel consumption; (3) much longer service intervals and life expectancy. Other aspects to consider include the reduction in through hulls (both props shaft and cooling water intake are externalised), the freeing up of a large amount of under bed storage space, and - although this will be a minor point - the cachet and talking point of visibly owning a twin set of rare and unusual engines.

On the other hand, using outboard alternators to charge your house batteries is slightly more of a challenge compared with inboard engines, although absolutely doable (the OXE engines have 130A alternators).

It goes without saying that maintenance and repair of outboards on a compact cruiser such as yours is generally so much easier than of inboard engines, and if done one-at-a-time off the transom doesn't even fundamentally interfere with boat usability ...

Each diesel OXE has a performance curve comparable to a petrol engine with roughly 30 percent more nominal horse power. The initial purchase cost of these outboards is approximately twice that of petrol engines but they actually amortise relatively quickly if used more than occasionally and over a number of years.

I am seriously considering buying a diesel outboard myself for my small electric cat (to be mounted in the centreline cockpit floor lazarette), for the above mentioned reasons.