370 lb RIB dinghy on Kato davits.... too much weight ?

[Anuska]

Boat (Island Packet 380) came equipped with a pair of Kato standard "Island" model davits, capacity of 300 lbs each. (600 lbs total) I'd like to go with the 'Ultralight' Highfield 310 rigged with Tohatsu 15, electric start, remote steering. At 370 lbs total rigged weight, in theory, the Kato davit on the motor end, taking the most load, would be within rated specs.

Having said that, I worry 370 lbs would be too much strain on the boat railing that holds the davits.. so safest bet is to go with the “non rigged” version with 5 hp OB at 160 lbs total weight. But the heavier "rigged" 310 sure would be a fun dinghy….so I wonder. Thoughts ?

 

[Hale Moana]

Each davit won't be holding 370 lbs. To figure out the weight on each end rest one end on a block of wood and the other end on a bathroom scale. You should make it so the dinghy sits level. The bigger problem will be the effect of that dinghy hanging off the stern of your boat. A friend of mine hangs a big heavy dinghy off the stern of his Shannon 38. It is very stern heavy. So he repainted the waterline stripe and you don't notice that that his Shannon is 6" down by the stern.

 

[Anuska]

The bigger problem will be the effect of that dinghy hanging off the stern of your boat.

Yeah I worry about that as well.

 

[kent_island_sailor]

Kato is a great operation. I would call them and ask

 

[kent_island_sailor]

FYI

Kato Marine

Davit - home page for Kato's davit website. Davits in different styles and sizes. Also antenna mounts, wind generator poles, arches and more.

katomarine.com

 

[MaxDog]

Take the motor off when raising in the davits?

 

[Anuska]

Take the motor off when raising in the davits?

I presume the motor on the rigged dinghy is bolted in place. Plus, disconnect of remote steering mechanism and starter wiring would make that impractical

 

[accnick]

Boat (Island Packet 380) came equipped with a pair of Kato standard "Island" model davits, capacity of 300 lbs each. (600 lbs total) I'd like to go with the 'Ultralight' Highfield 310 rigged with Tohatsu 15, electric start, remote steering. At 370 lbs total rigged weight, in theory, the Kato davit on the motor end, taking the most load, would be within rated specs.

Having said that, I worry 370 lbs would be too much strain on the boat railing that holds the davits.. so safest bet is to go with the “non rigged” version with 5 hp OB at 160 lbs total weight. But the heavier "rigged" 310 sure would be a fun dinghy….so I wonder. Thoughts ?

Talk to the manufacturer of your davits.

There can be a fair amount of difference between static loading and dynamic loading on davits. You also have to consider the way they are mounted. Are they surface-mounted, or socketed through the deck edge to something below? (Think of it like a deck-stepped mast vs a through-stepped mast chocked at the decks.)

These davits on my powerboat go through the cockpit coming to through-bolted sockets on the cockpit sole. What isn't obvious is that the plates on deck have a tubular spigot on the underside that fits tightly around the primary structural member of the davits--no slop, so it's like a properly-chocked mast.

Even so, I had to have these davits totally re-built shortly after these pictures were taken due to a couple off stress cracks. They are 25 years old, and have had heavy use, including dinghies heavier than my current rib, which is probably about 225 pounds all up when you add engine, anchor, lifejackets, oars, etc., but not the fuel tank, which comes out.

Another thing you need to do is really secure the dinghy against movement under way. I secure mine with straps around the dinghy to the davit frame, as well as transverse tackles that prevent the dinghy from swinging in any way.

 

[Zonker]

Not sure why you'd need a console, electric start on a 15, and remote steering on a 3.1m dinghy. That seems like it would take up a lot of space and just add cost and weight.

Highfield Classic 310 - 137 lbs
Tohatsu 15 - 95 lbs
3 or 6 gal fuel tank - 5 lbs
6 gal gasoline - 36 lbs
Total - 273 lbs

Which should be fine for your davits. Pulling outboard motor pull handles builds character.

If you ever have to drag it up a beach you will appreciate the 100 lbs less.

(Also - dinghy manufacturers, like many boat manufacturers may be a trifle "optimistic" about actual as-rigged weight.)

 

[accnick]

Not sure why you'd need a console, electric start on a 15, and remote steering on a 3.1m dinghy. That seems like it would take up a lot of space and just add cost and weight.

Highfield Classic 310 - 137 lbs
Tohatsu 15 - 95 lbs
3 or 6 gal fuel tank - 5 lbs
6 gal gasoline - 36 lbs
Total - 273 lbs

Which should be fine for your davits. Pulling outboard motor pull handles builds character.

If you ever have to drag it up a beach you will appreciate the 100 lbs less.

(Also - dinghy manufacturers, like many boat manufacturers may be a trifle "optimistic" about actual as-rigged weight.)

Especially when you add stuff like oars, pumps, PFDs, anchors, etc.

 

[low bum]

And I wonder about the dinghy half or 3/4 full of water. My little plastic fantastic is upside down on deck - but do the RIB/davit people every worry about big waves? They don't seem to, as often as this technique seems to be employed.

 

[accnick]

And I wonder about the dinghy half or 3/4 full of water. My little plastic fantastic is upside down on deck - but do the RIB/davit people every worry about big waves? They don't seem to, as often as this technique seems to be employed.

If it's going to rain, or if I have to travel on a rough day, I just pull the transom plug out of my dinghy on davits. A dinghy can hold a lot of water.

 

[Fah Kiew Tu]

If it's going to rain, or if I have to travel on a rough day, I just pull the transom plug out of my dinghy on davits. A dinghy can hold a lot of water.

Yeah nice idea - IF you can reach the bung when the dinghy is raised. I can't.

So yes I am very conscious of the amount of water in the dinghy when it's hoisted. I'd like to design a good cover but the falls make that problematic. So far I haven't come up with a solution. I suppose I should fit another bung that I can reach...

FKT

 

[accnick]

Yeah nice idea - IF you can reach the bung when the dinghy is raised. I can't.

So yes I am very conscious of the amount of water in the dinghy when it's hoisted. I'd like to design a good cover but the falls make that problematic. So far I haven't come up with a solution. I suppose I should fit another bung that I can reach...

FKT

Yeah, reaching the bung can be problematic. I have to hang onto the davits with one arm and lean over and reach down with the other, which puts most of my weight on the davit arm that is already carrying at least 70% of the total dinghy weight. That's another reason to go overkill on davit strength.

 

[Fah Kiew Tu]

Yeah, reaching the bung can be problematic. I have to hang onto the davits with one arm and lean over and reach down with the other, which puts most of my weight on the davit arm that is already carrying at least 70% of the total dinghy weight. That's another reason to go overkill on davit strength.

Agree completely. That's why I had an engineer do a FEA on what I proposed to build for my boat. The upshot of that was, I went up a pipe size and added more stiffeners where recommended. I wanted a very robust safety margin. One of the commercial offerings seemed to have very little margin.

FKT

 

[Zonker]

Just pull the bung before hoisting while you are in the dinghy hooking up the falls. I remembered to do it at least 3/4 of the time. But I could reach it with a big stretch

 

[Panoramix]

I'd like to go with the 'Ultralight' Highfield 310 rigged with Tohatsu 15, electric start, remote steering. At 370 lbs total rigged weight, [...]

Wow, you guys take your dinghies seriously!!!

A 6HP engine is more than enough, it will keep the boat planing with at least 2 on board, will bring lines around the harbour and even push around the big boat... and weight way less! I don't get why you would want remote steering, it just adds complexity for no real benefit. Also the weight of the whole thing that far aft might well make the main boat hobby horse badly (pitching moment of inertia increased) and will certainly act as a brake in light wind conditions! 360lbs is a lot for a dinghy, that is just like asking two big guys to stand on your swimming platform while you are sailing. Also if you are landing in a bit of surf, you will need to be at least 4 on board to carry the boat up before the next wave... And that's assuming you can each carry 45 kg (90lbs) which is the weight of a bag of cement before they reduced them to 25kg (50lbs) for health and safety reasons...

But that's just me...

 

[El Borracho]

.....But the heavier "rigged" 310 sure would be a fun dinghy….so I wonder. Thoughts ?

Fun? Seek help. Trade the Island Packet for a houseboat.

 

[toddster]

I wanted a substantial enough dinghy for going say, five miles or so from anchorage to scuba sites. Use case: two divers, two rigs, maybe even a third to act as tender. Turns out that's also about the distance I've been going for groceries. But yeah - Zodiac RIB 120 lbs, 9.9HP 4-stroke 95 lbs. (already had), fuel&battery&misc crap maybe 50 lbs. Say 250-275 lbs all up.

I've seen a couple of them fitted out with console and padded seats and that takes up pretty much the whole boat - no space left for cargo. But I guess it depends on your use.

Planes like a dream with 300 lbs cargo. The motor does have the Doel fins, which I got for my old HP floor dinghy. (SO glad I upgraded to the RIB.) Some slow day, I need to take the fins off and see how the RIB does without them. If there's no cargo, I usually sit forward and use a tiller extension (but I can reach, somewhat uncomfortably, without it.)

ANYHOW, re: pulling the bung. A few weeks ago, I decided I was tired of bailing out the dinghy every morning (it's like, winter here) and picked up a little tiny Rule automatic bilge pump that fits right down in the slot next to the bung. They're really small these days. Already have a battery on board for the motor & the fish finder. Every morning I glance into the dinghy, by habit, only to find it high and dry, no matter how much it rained last night. Feels quite decadent.

I was worried about the pump running the battery down (might have to *gasp* pull the rope some morning.) But so far it hasn't managed to, even if the motor doesn't run for a week. But I did order a little solar trickle charger. Haven't figured out how to mount it yet. Prolly just bungee on the back of one tube. And as long as we're going this far, a little fuse box & switch panel (with 12V & USB charging ports) to install inside the battery box.

 

[Anuska]

Re space, the Highfield 310 Ultralight rigged version has storage under the seat (water tight and lockable) and no console in the conventional sense … the steering box is suspended by one aluminum tube so your legs go under it. So, yes, less room than the non rigged version but not much less.