Outboard HP vs Dinghy Design

[sailorboy1]

My dinghy is 2 years old and on a slow death I think. I also have a 6 HP outboard because that was in the limits of the dink I had before. The combo seems to be a lot slower that others and it makes be envious. So I have been thinking the "down the road" solution some.

I could change out my outboard to a 10 HP. But, the other day a guy flew by me on a hard bottom inflatable dinghy and it had the same outboard as I have. My outboard runs just fine, so hate to spend $2k plus to just get a bigger one if not really needed.

Is a RIB really is so much faster than a soft bottom?

Would a 10 HP on a soft bottom be more or less performance than a 6 HP on a hard bottom RIB?

[Suijin]

Yes it is that much faster, and quicker to plane with a given horsepower.

The downside is weight and stowability. I have a fiberglass 10’ hardbottom and believe it or not with just me in it (200 lbs) and my 2.5 hp four stroke it will plane in the right conditions. With my 8 hp two stroke it’s downright dangerous going flat out.

That said at @ 110 lbs it’s a chore to hoist up on deck single handed. Fortunately mine has a folding transom so it’s very low profile when deflated on the coach roof for offshore work.

Every dinghy is a fistful of compromises. The one you pick is a function of which compromises best fit your needs.

Regarding motors I have two because the 8 hp is a bear to wrestle into the dink without dropping it in the drink. I only put it on when going longer distances and exploring.

As for your specific question I think they might be generally comparable, pending the specifics of the boats and the motors. But the hardbottom is going to handle better. Not sure either will get up on plane with any appreciable load though.

[Kenomac]

Side Note: We always attach a free halyard or davit lift line to haul the 15hp Mercury 2 stroke outboard on and off or around the boat. Makes lifting it a breeze and negates the possibility of it going for a swim.

Yes a hard bottomed dinghy planes much easier, but we were also very pleased with the performance of our cheap Costco Hydro-Force $599 dinghy this season. Our Highfield is excellent, but for dollar value you can’t beat the Hydro-Force.

[sailorboy1]

Well My current setup gets up on plane if it's just me in it. But it doesn't go really that much faster because it gets so squirrely that I have to back off.

The weight isn't that much an issue. I keep it on my davits with the engine on so as long as the total weight is under the limit it doesn't matter.

[jeff356]

Glad you raised this question as I am under the same scenario.
Have an Achilles SPD-290 with plywood 5 part floor but soft bottom and a Tohatsu 6hp 4 stroke. I can get on plane and scream cross the water by my self but that is it. Add more people and we slog along.

Contemplated bigger engine, maybe a 9.9hp but maybe another dink with a hard bottom, either aluminum or glass is the answer. Also would a little longer dink, perhaps a 310cm work better.

On the dink usage we normally inflate in the beginning of the season and store on the fore deck upside down with a cover to protect it or on the davits when traveling and out.

Will be interesting to see the replies and other input.

Jeff

[Kenomac]

Originally Posted by sailorboy1

Well My current setup gets up on plane if it's just me in it. But it doesn't go really that much faster because it gets so squirrely that I have to back off.

The weight isn't that much an issue. I keep it on my davits with the engine on so as long as the total weight is under the limit it doesn't matter.

Our Highfield aluminum bottom is rock solid up on a plane. The Hydro-Force soft inflatable bottom with aluminum inner floor is very "squirrely" up on a plane with just the two of us, and I usually have to back off to maintain control. Both dinghies have 15hp 2 stroke Mercury outboards. The Hydro-Force will plane with up to 350lbs of human aboard, but it's not a comfortable feeling: any more and it's a displacement hull.

Sailorboy, My recommendation is to look at the Highfield enhanced PVC model dinghy. It's light weight and will probably plane with your current outboard. Or upgrade to a 9.9 or 15hp outboard with the Highfield and keep your smaller outboard as a spare.

[Peeew]

My AB aluminum bottom dinghy planes well with one or two on board. I decided on a larger outboard (9.9 HP) thinking that in some rare case it might be better if I needed to push or pull my boat with it. It is the same weight as the next lower power engine so it seemed ok. But, it is heavy at 87 lbs

[S/V Alchemy]

Originally Posted by Suijin

Yes it is that much faster, and quicker to plane with a given horsepower.

The downside is weight and stowability. I have a fiberglass 10’ hardbottom and believe it or not with just me in it (200 lbs) and my 2.5 hp four stroke it will plane in the right conditions. With my 8 hp two stroke it’s downright dangerous going flat out.

That said at @ 110 lbs it’s a chore to hoist up on deck single handed. Fortunately mine has a folding transom so it’s very low profile when deflated on the coach roof for offshore work.

Every dinghy is a fistful of compromises. The one you pick is a function of which compromises best fit your needs.

Regarding motors I have two because the 8 hp is a bear to wrestle into the dink without dropping it in the drink. I only put it on when going longer distances and exploring.

As for your specific question I think they might be generally comparable, pending the specifics of the boats and the motors. But the hardbottom is going to handle better. Not sure either will get up on plane with any appreciable load though.

Our experience backs this up pretty strongly. We had a 9.9 Honda BF100 with a Doel fin on a Zodiac Yachtline C310, a RIB of some popularity and in which I could sluice along at 22 knots on flat calm days on the plane. But it rowed poorly and the 120 pound RIB and 90 pound motor were a hassle to handle; in fact, one of my davits snapped underway with only the RIB on them and I nearly fouled our prop with the falls and associated lines in the water. Hauling the thing on deck was also a chore, with its windscoop qualities and the poor effect it had on visibility.



So we reviewed our options, parted with the Honda 9.9 (for a good price as it's a good outboard) and went with two tenders, a nesting 10 footer suitable for rowing and sailing, and a folding 10 foot Portabote with an air-cooled Honda 2 HP, now available in 2.3 HP and, at 27 pounds' weight, is light enough for my miniature wife to one-arm on and off the deck and onto the transom. The nesting dinghy is for "fun" and people moving, and the Portabote is for cargo, fuel jugs, laundry and whatever else might stain it as it is very tough and yet folds down to surfboard size and is lashed to the starboard rail. This suits us because two tenders at "slow" speed are better for our three-person crew than one RIB capable, when unloaded, of going on the plane, which is enjoyable but not what we need of a tender. YMMV, but we've been content to date with our decision, and I can see without standing on my toes from the pilothouse helm. More here: https://alchemy2009.blogspot.com/201...r-moments.html

[a64pilot]

Before anybody goes to buy a bigger motor, put a tach on it and see if it’s propped correctly.
Many times they are not.
There are cheaper ones, but I bought this one as it is waterproof, has a replaceable battery and is also an hour meter so so can keep up with servicing
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Ribs are much faster than soft bottoms, and a full hard boat like a tinnie is faster than a rib and needs less motor, those tubes dragging through the water is a lot of drag.

Even one RiB can be faster than the others, my 3.10 Zodiac was honestly a 30 kt Boat with my Suzuki 20 HP and a Solas 11 pitch SS prop.
Identical motor and prop on my newer AB 10’ RIB is 10 kts slower, the Zodiac was much heavier. Too bad it fell apart though.

[Oceanride007]

I thought I Planned well, to replace my Walkerbay with a deceased 2hp 2/s, I went for a 2.6m Barefoot made in Brisbane, Aus, No doubt quality is very good, no issue there have sun cover over tubes and shelf for Anchor/Fuel wide wheels that fold down, but am sure that anticipated weight of 32Kg was way too optimistic. Fitted a 6hp 2/s Tohatsu, think in small sizes 2/s is better if you can get em.Planes nicely with one person and minimal gear.


Anyway I wanted a bigger motor, manufacturer said "no" because of Transom force. And if the beach is soft or Mud can't handle it myself. Extra weight on Davits required me to strengthen backing plate, Dyneema strops, a mono rail Davit to get the Motor off the Transom. Yet to get is a smaller Motor, Think I'll also over carb my 6hp Tohatsu,

Its all a compromise, do you know you can't deflate a Rib, I didn't. Slightly bigger Dinghy will plane easier but I didn't want it overhanging my Yacht Transom. A soft Dinghy will sag in Davits.

[jimbunyard]

Never owning a pneumatic boat, does the inflation pressure make a difference in speed and handling? Is it something that is adaptable to conditions, like tire pressure, or is it pretty much a 'one setting for all conditions'?


To the uninitiated, it seems that the higher the pressure the better, as well as that there could be huge differences between (soft bottom especially) dinghies' construction that could have correspondingly large differences in performance.

[Thumbs Up]

A lot of people that have hard bottoms keep them soft. Once it is on plane it makes no difference, I have an old grey hypalon rover (soft floor with plywood inside) I keep it hard, makes a big difference. It also has an inflatable keel which makes it handle better but gives it a vee bottom so actually planes less. (the keel has a leak so I don't use it) With my 8 hp two stroke with fins it jumps on plane very fast and planes level even if I sit on the transom. (so don't give up on your soft bottom just yet) The doelfins make a big difference! All of the hard bottom dinghy responders so far have pretty light hardbottoms. (aluminum floors). Fiberglass bottom dinghys are usually very heavy and need a 15 to be able to plane with passengers. When mine is running good it will plane with 3 people. I would rather have the 9.8 2 stroke, it weighs the same but would really like an aluminum bottom inflatable with a either a 9.8 or an 18 hp 2 stroke which weighs the same as the 9.9. We are talking Tohatsu (Nissan). I doubt that your dinghy is worn out after two years, maybe it wasn't really designed to plane. You can also buy high pressure inflatable floor dinghys that work really well (until they don't)

[Jim Cate]

Its all a compromise, do you know you can't deflate a Rib, I didn't.

Not sure what you mean here, but we routinely deflate our Aluminium hull Gemini RIB for foredeck storage at sea. Couldn't have dealt with a 3.5 m dink otherwise.

Jim

[Oceanride007]

Hi Jim, Was told by manufacturer that they occasionally do transport a RIB deflated. But once in the hands of owner its too much risk. Its the glued joints around Transom to Tube that would appear to be the problem.

Think I have seen like a socket arrangement that holds the Transom on some, but I imagine Thrust limit on those would be low, it would look funny wouldn't it. Anyway maybe on a Rigid RIB like mine maybe a little deflation would be OK. Still apparently you get away with it.

[Nicholson58]

We have a 9.5 Carib RIB with 15 HP merc 2 stroke. very quick. Most Caribbean locations demand fast capabilities with long transports and longer excursions.

The GoTo dinghy is an AB aluminum RIB.