Planing Dinghy: Hp vs Dinghy type vs #Passengers

[Adelie]

Originally Posted by chris mac

Just adding new info as we have used the new 3.1 m Achilles more.
With 3 adults(roughly 550 lbs) it gets on plane fairly easily.
Same 3 adults plus additional 100lbs of gear in the bow, it did get on plane but struggled. I believe it would get up with 4 adults shifting weight as needed.

HP???

[rslifkin]

As an update to this, we finally mounted an outboard (Nissan/Tohatsu 6hp) on our dinghy (12 foot Starcraft skiff). Being that the outboard needed a new prop when I bought it, I swapped to a 7.8x7 (stock is 7.8x8) figuring it's likely to need the lower pitch to plane with 2 of us and some stuff.

Haven't gotten to test run the thing with 2 people yet, but with just me I topped out at 13.6 kts after backing off to stay just short of the rev limiter (it's a little underpropped with 1 person). Trim was definitely not ideal, as I had the outboard in the lowest trim hole, so I was definitely wasting power to hold the bow down (and it needed nearly pretty much WOT to stay on a clean plane with me on the back seat, fuel tank in the middle and basically no other weight). If I slowed down more than a tiny bit from WOT the bow went way up. But with it being slightly underpropped, it did take off great. Starting from ~4 kts, snapping the throttle open was immediate acceleration, pretty much bow comes up, boat accelerates, count to 3, bow drops and it accelerates slightly more as it winds towards the rev limiter.

I'm going to add a longer fuel line and move the tank all the way forward and see if that lets me run solo without keeping the outboard trimmed in all the way. 1 notch out on the trim and appropriate weight distribution should cut the prop slip down a bit and I expect to see at least 14 kts solo, maybe a little more (and it would still be faster with a higher pitch prop, likely 15+ kts).

Hopefully I'll get a chance to run it up on plane with both of us in the dinghy soon. I'm estimating we'll get somewhere in the 12 - 13 kt range. Considering how quickly it accelerated with 1 person and the lower pitch prop, I don't expect any trouble getting on plane with 2 people as long as the hull can plane with that amount of weight without needing more speed than a 6hp can deliver.

[S/V Selene]

I have a 9.5 Defender 300 RIB with a 6HP Tohatsu.


I cannot get this thing to plane under any circumstance.


I'm approx 240#'s and I have the seat as far forward as it will go and I cannot get up on plane.


When wife and I are on and she sits on bow and I'm forward as muuch as I can she comes up a little but will not plane.


It was suggested by the salesman at defender that I put a hydrofoil on the shaft of the motor or trim tabs.


Any thoughts about these options?

[j.g.evans]

I now have a 10' RIB with a 5 HP and weigh 175 and can just get her to plane.

At an earlier time I had 15 HP 2 stroke. It weighed about the same as your engine. I was able to get a 10' RIB to plane with 3 adults with the 15 HP. With just me I was able to go more than 25 knots. This was a lot of fun. Look for a used one.

[rslifkin]

I did some more testing with our dinghy through the season. It's slower than I'd hoped, but good enough. After adding a set of smart tabs, minimum planing speed is much lower and it'll plane a heavier load. I may still need to experiment with adjustment on the tabs a bit more.

With the 6hp Nissan / Tohatsu, the best all around prop seems to be the 7.8x7. It's good for about 12 kts solo once you back off to stay off the rev limiter, 9 - 9.5 kts (and on plane) with 2 people. A little slower and once the dog is added, but it does still plane. It'll make 8 kts with 2 people against a 25 kt headwind and 6 - 8 inch chop. At that point the wake showed it as just short of planing.

I tried a couple of other props with worse results. A 7.8x8 is faster with 1 person (13 kts), but is a bit overpropped for a heavier load. If I could get a 7.5 pitch, that would probably be ideal (the 7 pitch has a bit of RPM headroom with heavier loads and is significantly underpropped for 1 person). The high thrust prop is all around worse (unless I were using the dinghy for something where low speed thrust and strong reverse mattered). It's no faster to plane, although it does plane at slightly lower speeds. Runs just shy of 12 kts with 1 person, but only 8.5 - 8.7 with 2 people. RPM-wise, it turns about the same as the 7.8x8 with 1 person, slightly more with 2 people. Doesn't hold speed in a chop or headwind any better either (actually slightly worse as the engine isn't revving as high).

I did buy an 8.5x7 prop for an 8/9.8hp Tohatsu, but I haven't managed to make it fit yet. It physically clears the anti-vent plate and the splines match up, but the center hub is further back in the prop. So I'd need spacers for the thrust surface (easy enough) and would need to shave the hub down or something to get enough thread exposed to properly install the prop nut. I'm thinking that prop may be a better match if it'll fit, as it's got about as much blade area as the high thrust prop, but without the thick, symmetrical, inefficient blades, so it should take less power to spin at higher speeds. Not sure if I'll actually come up with a way to make it fit or not though.

[Adelie]

Originally Posted by S/V Selene

I have a 9.5 Defender 300 RIB with a 6HP Tohatsu.


I cannot get this thing to plane under any circumstance.


I'm approx 240#'s and I have the seat as far forward as it will go and I cannot get up on plane.


When wife and I are on and she sits on bow and I'm forward as muuch as I can she comes up a little but will not plane.


It was suggested by the salesman at defender that I put a hydrofoil on the shaft of the motor or trim tabs.


Any thoughts about these options?

Your best place to start would be slightly flatter prop. Whatever the current one is get one 1" less in pitch.

I doubt that will get 2 of you on a plane, but it might. It will cost you slightly in top speed, but if it's the difference between getting on a plane and not....

Wings and a new prop may get you and your wife on a plane.

Let me know what happens.

[Adelie]

Originally Posted by rslifkin

I did some more testing with our dinghy through the season. It's slower than I'd hoped, but good enough. After adding a set of smart tabs, minimum planing speed is much lower and it'll plane a heavier load. I may still need to experiment with adjustment on the tabs a bit more.

With the 6hp Nissan / Tohatsu, the best all around prop seems to be the 7.8x7. It's good for about 12 kts solo once you back off to stay off the rev limiter, 9 - 9.5 kts (and on plane) with 2 people. A little slower and once the dog is added, but it does still plane. It'll make 8 kts with 2 people against a 25 kt headwind and 6 - 8 inch chop. At that point the wake showed it as just short of planing.

I tried a couple of other props with worse results. A 7.8x8 is faster with 1 person (13 kts), but is a bit overpropped for a heavier load. If I could get a 7.5 pitch, that would probably be ideal (the 7 pitch has a bit of RPM headroom with heavier loads and is significantly underpropped for 1 person). The high thrust prop is all around worse (unless I were using the dinghy for something where low speed thrust and strong reverse mattered). It's no faster to plane, although it does plane at slightly lower speeds. Runs just shy of 12 kts with 1 person, but only 8.5 - 8.7 with 2 people. RPM-wise, it turns about the same as the 7.8x8 with 1 person, slightly more with 2 people. Doesn't hold speed in a chop or headwind any better either (actually slightly worse as the engine isn't revving as high).

I did buy an 8.5x7 prop for an 8/9.8hp Tohatsu, but I haven't managed to make it fit yet. It physically clears the anti-vent plate and the splines match up, but the center hub is further back in the prop. So I'd need spacers for the thrust surface (easy enough) and would need to shave the hub down or something to get enough thread exposed to properly install the prop nut. I'm thinking that prop may be a better match if it'll fit, as it's got about as much blade area as the high thrust prop, but without the thick, symmetrical, inefficient blades, so it should take less power to spin at higher speeds. Not sure if I'll actually come up with a way to make it fit or not though.

What dinghy? I don't want to go back searching thru all those pages.

[rslifkin]

Originally Posted by Adelie

What dinghy? I don't want to go back searching thru all those pages.

It's a 1960s build Starcraft 12 foot aluminum skiff. 57" beam, about 150 lbs empty weight (so about 250 with outboard, fuel, oars, etc.).

[Cheechako]

Originally Posted by S/V Selene

I have a 9.5 Defender 300 RIB with a 6HP Tohatsu.


I cannot get this thing to plane under any circumstance.


I'm approx 240#'s and I have the seat as far forward as it will go and I cannot get up on plane.


When wife and I are on and she sits on bow and I'm forward as muuch as I can she comes up a little but will not plane.


It was suggested by the salesman at defender that I put a hydrofoil on the shaft of the motor or trim tabs.


Any thoughts about these options?

My opinion is you simply need more HP. You are heavy. How much does that RIB weigh? You can get a outboard handle extension (or PVC pipe of the right size) and experiment with just you sitting forward.
As far as tabs go.... how close are you to planing now? If you are very close then maybe. Otherwise, dont waste your money.

[Adelie]

More HP would do it but there would be tradeoffs. A motor with more ho would involve:
A. More $ for replacement of the existing motor.
B. It would be heavier making it significantly more difficult to mount and dismount in the dinghy, to store on the mothership and somewhat more difficult to drag up a beach.
C. Physically larger which may be marginally more difficult to store.
D. New spares would need to be obtained and paid for. And slightly updated maintenance procedures.

The point of this thread is for members to be able to make a reasonable guess of what the bare minimum engine they need to plane on the dinghy they have or visa versa.

Selene’s dinghy is very marginal for that size engine. Wings, flatter prop, tiller extension, 3gal fuel tank in the bow may all be required to plane. All of these the cheap low hanging fruit that may make it possible. Or it may not be possible.

[rslifkin]

With my dinghy adding the smart tabs made a massive difference in planing ability. You keep weight a bit further aft than you would otherwise (being that you're only adding lift to the stern), but the added lift reduces minimum planing speed massively and also increases load carrying capacity on plane. It won't necessarily improve speed when lightly loaded though and because of added drag, it might make it worse too.

Experimenting with outboard mounting height is worthwhile as well. On my setup, raising the outboard about 3/4" on the transom added almost a half-knot with most of the props I tried (fastest trim position did vary based on mounting height).

[Jammer]

So, conclusions?

- Planing performance is mainly about combined weight of the boat, motor, and people, and less about boat shape and construction.
- 6hp provides marginal planing performance, for one person and a small dinghy
- 8hp not much info, in the USA these are all derated 9.9hp and no one buys them.
- 9.9hp provides adequate performance for two people in a small boat.

Since weight is a primary factor, I offer this rundown of what is available:

9.8/9.9 HP
Tohatsu 2-stroke 57 pounds
Tohatsu 4-stroke 81 pounds
Yamaha Enduro 2-stroke 84 pounds
Honda 4-stroke 92 pounds
Yamaha 4-stroke 92 pounds
Suzuki 4-stroke 97 pounds

15 hp
Yamaha Enduro 2-stroke 84 pounds
Tohatsu 2-stroke 90 pounds
Tohatsu 4-stroke 95 pounds
Honda 4-stroke 104 pounds
Suzuki 4-stroke 105 pounds
Yamaha 4-stroke 132 pounds

18 hp
Tohatsu 2-stroke 90 pounds

20 hp
Tohatsu 4-stroke 95 pounds
Honda 4-stroke 104 pounds
Suzuki 4-stroke 105 pounds
Yamaha 4-stroke 146 pounds

25 hp
Yamaha Enduro 2-stroke 117 pounds

[Adelie]

Hull shape and construction are very important.
Moderate changes in the rear end of a hard dinghy is the difference between a good rowing dinghy and a planing dinghy.

For inflatables, soft floors vs air floor vs slatted floor vs RIB have a significant effect on planing. Soft floors automatically deform to the most drag inducing shape, slatted or plywood floors are significantly better, better still is an air floor with a keel shape or a keel tube under the slats which provide yaw stability though it is not clear if there is an improvement in planing. Best is the RIB

[Jammer]

Originally Posted by Adelie

Hull shape and construction are very important.

I agree that should be true, but the data in this thread doesn't bear it out, at least not among vessels intended for planing (tinnies, Gatehouse's GV-10 wood planing dinghy, HP floor inflatables, and RIBs) that are discussed here.


I think that whatever differences exist aren't great enough to make a 6 hp outboard feasible for planing use, and similarly aren't great enough to make a 10 hp outboard unworkable for two people.



At one point you said your purpose for posting the thread was to determine the best advice to give budget-minded US-based cruisers starting a voyage and intending to spend considerable time in the Caribbean. I think good choices would include a 2-3 hp motor that can be sold or left behind when leaving the USA, or a 9.9 or 15 hp Tohatsu 4 stroke, as these are light enough to work out OK. Parts availability will be poor outside USA/AU/NZ but the astute sailor can purchase relevant parts before leaving as they are not expensive and take up little space. I would carry a complete carburetor, a water pump, a thermostat, a fuel pump, an oil pump, and some impellers and a few oil and fuel filters. The oil filters and spark plugs are widely used in automotive applications and are available anywhere. Props interchange with other common makes. That should be enough parts to get through most of the useful life of the engine.

[rslifkin]

With our setup I'd say the 6hp isn't necessarily ideal, but it's certainly workable for planing use. It'll plane a 400 lb load beyond the basic stuff in the dinghy, but at that load planing means running the thing wide open and it's still not super fast. Much faster than a non planing setup with a 2.5hp would be though (probably close to double the speed). As-is due to the hull shape of our dinghy I've had to slow down when running solo in a chop to avoid bashing the dinghy (and my spine) to bits. With 2 people I've yet to need to back off, so a bit more power would be fine there.

Something around 10hp would certainly be better (would plane with a heavier load, would go significantly faster with our current max planing load and would fly with 1 person). But at the same time, it would likely burn more fuel (which means needing to carry more fuel) and it would be another 25 lbs added to the heavy end of the dinghy making it more of a big deal to hoist in davits with the outboard mounted (I won't suffer the downsides of a 2 stroke to get more power at our current weight).