Sea Sled madness. It’s in my brain.

Wait a couple of months and you'll see the real deal

 

Well, I'm not just a scarred knuckle wood-butcher. I'm a scientist and engineer at heart. I want to see exactly. Could it be as simple as a zero deadrise hull that doesn't pound?

 

Ignore CFD then!!

As noted yeee shall get what yeee desires exactly... but in a few months time!!

 

I'm at the (a) slow part, and so naturally my brain is whirling like a squirrel in a hamster wheel.

I'm thinking about stern drives vs outboards. I'm free to think this because the designer didn't spec anything. I can fit an Alpha 1 G2, and its 4.3l on either side between the transom and the monster B6 bulkhead. A heck of a lot cheaper, but also much heavier and takes up all that back deck space. On the plus side I can work on a 4.3l V6 easier there than I can work on an outboard.

Thoughts?

 

Can you post a photo of the transom for reference please John?

Possibly another positive advantage could be that you would get more torque from a sterndrive unit than from an outboard motor?
How do the reduction ratios on the engines compare?
I think that for your Sled the more torque that you can get, the better, especially when you will be heavily loaded.

One disadvantage though of outdrives is their service interval (ignore this at your peril), and the consequences of weed and shells growing inside them, and causing havoc.
I think the service interval is usually around 100 hrs (?) - and if the boat is in use every day pretty much, these hours will add up very quickly.

Edit - I did a bit of googling, and they are suggesting that 300 hours is a reasonable service interval for outdrive legs. And if you are doing say 25 hours a week, that would be every 3 months that you need to haul out.

 

Many. But this is the kicker...

Everything on a boat is about maintenance and ease of access.
Thus, if this ticks your box...then why add complications into the mix, simply because you can? KISS

Just think of the hours (and money) you save not having to wait for someone to help you establish what is wrong when it does go wrong?

 

And, you need to hang drive shafts, rudders, have water strainers for cooling.
Lots more stuff than outboards (and no directional thrust).

 

DC, your decision making around this boat is starting to sound "ad hoc", you are chopping and changing ideas on the hop, the difference between twin outboard and twin sterndrives weight-wise could make a significant difference, if it turns out less weight at the stern, and less weight overall, are desirable.

 

I've been treated relatively well by my old stern drive, but I do find it a little more maintenance intensive than an equivalent outboard.

That said, I could replace the whole thing pretty regularly for the cost of a new outboard.

If you go that route, I personally would stay away from any drive that needs an intact boot to stay above water. A lot of the Merc stuff does, I know. My old Volvo 280 is a pig, but bulletproof, and you can test a boot without flooding the compartment.

It was also in commercial service for most of its forty years.

On the other hand...no power trim, which is unthinkable these days. So I'm not recommending it or anything. Just saying...they can be tough, they can be cheap...but they are big and heavy and I'd replace mine with an outboard if I were keeping the boat.

But I resealed it when I got it, and have traveled about 10,000km on it since then. I replaced the u-joints once. And installed a zerk so I could pump new grease into the bellhousing when I have it off the boat. Other than that... couple of fan belts, one starter, exhaust manifolds and oil filters. And that's on an old boat that's seen much harder use than your typical pleasure craft.

 

Thanks CR. That's some pretty solid experience.

BB, I meant an Inboard/outboard setup.

The I/o option, like all engineering decisions, has its pros and cons. Less than half the cost of equivalent power. But almost twice the weight. And a significant sacrifice of space. But I'm comfortable pulling wrenches on a V6 on engine beds. The guts of an outboard intimidate me.

 

How does the reduction ratio compare on a typical V-6 inboard and a V-6 outboard?

Here is the specification for a 250 hp Mercury 4.5 litre inboard - it is a heavy beast for sure, at 761 lbs excluding the gearbox.
Mercury® MerCruiser® 4.5L | Mercury Marine https://www.mercurymarine.com/en/us/engines/inboard-and-sterndrive/mercruiser/45l/#specifications

A Mercury 250 hp four stroke V-6 O/B motor weighs 527 lbs, and has a gearbox ratio of 1.75 :1.
https://www.mercurymarine.com/en/us/engines/outboard/fourstroke/175-300-hp/

 

There's a range of sterndrive ratios. Comparable to the outboard products from the same manufacturer.

Thanks for the weight figures @bajansailor . That 761 doesn't include, as you say, gearbox and leg, while the OB figure is everything. Big difference.

 

When Ad Hoc calculated your hydrostatics, did his figures give you the kgs / cm or lbs / inch immersion as well?
I am thinking that these figures should be relatively high for your boat?
So the extra weight of the I/O engines compared to the O/B motors might not have as dramatic an effect as one might think?
I am wildly guessing that an engine, gearbox and leg might be about 1,200 lbs, vs say 550 lbs for an O/B motor, so for two engines that would be 2,400 lbs vs 1,100 lbs - 1,300 lbs more.
How much would this extra weight sink Serenity?
And would it be feasible to have Bravo Two legs? These are the beefiest type available, with the most amount of torque - and I think that you will need as much torque as you can muster, especially if you have a heavy load of building materials on board.
Drives Bravo Two® | Mercury Marine https://www.mercurymarine.com/en/us/engines/inboard-and-sterndrive/drives/bravo-two/

There used to be a homebuilt power cat here (she now lives in Antigua, if she is still afloat) about 30' long - she started off life with 2 x 150 hp Yanmar diesel inboards with Hamilton water jets, no expense spared. And she could not even try to get up on the plane.
I think they ran her in displacement mode for a while before stretching her about 4', and putting pods on for a pair of massive Yamaha 300 hp 4 stroke O/B motors.
But I don't think these were very successful (other than drinking vast amounts of petrol), so then they stretched her by another 4' and went back to diesel inboards (these were now 225 hp each), and this time with outdrive legs.
But the boat was so heavy, with bluff bulldozer shaped bows, that they were breaking shafts, from trying to put too much torque through them when getting the boat up on the plane.
And I think they probably had Bravo Two legs.
Things improved a bit when they then added wave piercer bows on at the front end - this helped to reduce the bulldozer effect a lot.
I will add some photos of her later.
But the gist of the above ramble is to be aware of the torques that your engines will be trying to put out when Serenity is heavily loaded.

Edit - here are a few photos of the bulldozer cat that used to live here.

In her original length, the transom was just aft of the step in the sheerline behind the name.
Then she was stretched 4', and outboard pods added.
And then she was stretched another 4' re the lower aft deck.


A view on the port outdrive leg - the plastic pipes on the transom were for spraying cooling water on the legs while underway.



And a view on the funky wave piercing bows that were added later



When this vessel was in her Mark II version, with the 2 x 300 hp O/B motors, she was transporting the same number of snorkellers up the coast at the same speed as the cat in my avatar photo, but she was burning four times as much petrol to do so.

 

Bajansailor, certainly strange to see the sprinkler system, surely that was superfluous ? A slap in the face to Mercury engineering if that was helping.

 

And the "teeth" on the front end, completes the ensemble !