AB RIB/ outboard combo help

[kwf05001]

Our davits were custom built by an amateur welder. They've held up really well for 20 years carrying 200+lbs and I've occasionally climbed into the dink while it's hanging. I'm well aware that dynamic forces while pounding into a chop could significantly exceed the static load of me. I am also aware the weight hanging off the stern of our boat is detrimental to her performance. That said, we aren't heading offshore in the immediate future and if we were, I wouldn't carry the dinghy in the davits!
Like I said, I'd love the robustness of the aluminum, but I also like the idea of future-proof HP.
I don't expect anyone here to give me the "right answer" and very much appreciate the input so far!

[Muaddib1116]

That sounds impressive. Are you hard limited on that size dinghy? I wonder if the price savings on the 20hp over the 25hp would let you go up a boat size. Another possible combination to agonize over.

[ranger58sb]

Originally Posted by kwf05001

Currently trying to commit to a new RIB/outboard.

We want an electric start for my wife's elbows, ability to plane with 4 or 5 adults, max length 10'6". I'd prefer a yamaha, but in a 20hp the tohatsu is 26lbs lighter (and of course cheaper). With the 25hp the yamaha's weight makes it no contest.

Our top contenders are:
1. AB 10 VL with Yamaha 25

2. AB 10 AL with Tohatsu 20

If you haven't already, you might compare the (aluminum) Highfield CL models.

And the Tohatsu 20 and the Suzuki 20 are both similarly lighter in weight than other brands of comparable HP. Both fuel-injected, and I'd go with that in any case.

We had an electric start fuel-injected Suzuki DF15A previously and it worked pretty well. The Tohatsu has leap-frogged them a slight bit and today I'd probably prefer the Tohatsu but... our local dealer hasn't got any so we went with a 20 Suzuki this time.

-Chris

[SteveSadler]

The formula for planing more weight is flatter bottom, more planing area, more horsepower. So air floors help vs ribs. 12' rib will plane more than 10'. But surprisingly noone mentioned foils on the outboard. They are huge help in planing. Get the biggest one you can find. I made my own out of 1/4" aluminum plate 24" wide.

[Muaddib1116]

I did mention that I bought some to try this season, I have high hopes.

[kwf05001]

Steve-I have had foils on the last three outboards I've owned. While they definitely do something they've never made the difference between not planning and planning on a given boat with a given load.


Muaddib1116-Yes, hard limited by our Moody 34's beam. As it is the AB 10 (10'6") is going to hang such that its bow is beyond our beam just a few inches...



At the moment I am leaning towards the higher hp (so FG hull), but the aluminum hull would be nice if folks think 20hp is already more than enough...

[PhilC]

I have a Avon 340 with 20hp Honda.electric start and console. It will plane with 3 small adults (average 130lb) and really careful weight distribution. I think part of the equation is the short waterline length on these short ribs. IMO you will need a 25 with just the right prop to plane a 10’ rib with four ‘Full size’ adults.

[PhilC]

The other consideration is the torque curve of the outboard. My Honda 20hp is probably only producing 12-15hp while trying to get on the plane, the revs are held down significantly until the boat is fully planing. A finer pitch prop would possibly help in this situation, the PO put a finer than normal 4 blade prop to try to improve but it’s possibly still to coarse. Does 20knots flat out.

[JPA Cate]

Originally Posted by kwf05001

This would be so much easier if it was a fiberglass vs Aluminum debate or a 20 vs 25hp or a Tohatsu vs Yamaha, but it's a benefits of AL vs more hp vs cost vs supply issues confluence instead!

Actually, having a floor that stays dry is a benefit every time you might otherwise ruin going ashore shoes, every time you bring the laundry back home dry from washing it, every time you don't have to muck with drying out wet grocery bags because a little water sloshed and you don't want salt water hassles (permanent damp) in the main cabin. For some things you CAN tell by looking which will work out better for a family of 4.

As to engine size. We have always managed to plane 4 adults with our various dinghies. However, one guest was way over 200 lbs, and for him, we bought a spare prop with a slightly different pitch, so it would be easier, and that worked fine, and gave us a spare prop, too, which can be handy. Never had more than 15 hp. So I think the lighter, double floor dinghy with the 20 horse engine should be adequate for 4, if not 5. The bow locker usually means you'll board from the bow, but we prefer side boarding, and it is just something that you get used to.

You should be aware that hanging the dinghy off the stern in davits, is just a lot of weight aft and on a long lever, too, in terms of hobbyhorsing, and also having sturdy enough davits. Do not be surprised if you hobbyhorse more than ever before. We do not have davits, the o/b rides on a bracket on the port quarter, and we disinflate the RIB (aluminum), and tuck its skin under it, and lash it securely on the foredeck, between the staysail stay and the mast.

Good luck with it, everything on a boat is a compromise.

[contrail]

Glad a couple of folks mentioned props. i went from barely able to plane with three to planing easily with four just by going with a finer pitch. i don't understand the science, but i sure understod the result!

[kwf05001]

I added a poll. Lets see your votes!

[murofsresiurc]

We have an AB10 AL with the Tohatsu 20HP, and a Euro console. With three fullsize adults, plus gear, planing was not an issue. I'm sure adding a fourth adult would be no problem getting on plane. You can always change the prop pitch if it's a problem, props are about $65 IIRC.



Overall, we have been very pleased with this setup. The aluminum hull is very nice if dragging the rib onto gravel beaches is a consideration.

[FlyingScot]

As a previous owner of AB fiberglass 10.5 with bow locker we swapped to an AB AL9.5 aluminum with bow locker. I can say without hesitation to get the aluminum one.
The bow locker is also essential IMO. Easier boarding. Place to put things out of sight. We usually have our 3 gallon fuel tank in there too. We keep a 6 gallon one for longer trips.
I would never buy a dinghy without the false floor. Besides keeping most things you sit on the floor dry, it is also flat and feels safer for normal use.

As far as planing a lot of adults, longer is better. Also prop matters. You have to choose slower top speed, but planes easy with more weight, or faster top speed but only when you have 2-3 people on board. Our 9.5 is too short for 5 adults for regular use.

Both AB's used the same Yamaha Enduro 15. We keep three props on board. The orignal size, a faster and a slower one.

[paradox]

What kind of davits are you using? I am hesitant to put my Highfield 310 with 5hp outboard on my davits(engine 60lb). a 10 or 20hp engine is a lot of weight!

[MicHughV]

Bigger and more powerful engines are always nice....but comes at a price...namely weight and fuel consumption.
Trying to get a 20-25 hp engine on and off a dink will require some type of outboard crane...
My favorite engine of all time was a 8 hp Yamaha 2 stroke. I could pick it up with one hand. It would get 2 people up on plane and was more than adequate for most purposes.

Bigger engines may provide more power and speed, etc, but travelling at speed in a dink with 3-4 people, means people are usually sitting on the tubes, and in a chop, can be a rough, wet ride.