Re-design of 8 Foot Dinghy . . .

[bajansailor]

My goal is to have as light a dinghy as possible, but still have something durable.

A plywood dinghy would generally be much lighter than a fibreglass dinghy (and especially so re a foam cored fibreglass dinghy) - the plywood dinghy would be reasonably durable if it was skinned with cloth and epoxy.

 

[tpenfield]

A plywood dinghy would generally be much lighter than a fiberglass dinghy (and especially so re a foam cored fibreglass dinghy) - the plywood dinghy would be reasonably durable if it was skinned with cloth and epoxy.

Self-bailing would be a challenge without a cored floor. If I can keep the thing to 135 lbs, I'd be OK with it.

 

[bajansailor]

Self-bailing would be a challenge without a cored floor.

I am just wondering why you have this requirement for it to be self bailing?
I can see how it could be if you have a bilge and an extra floor higher up, but this extra floor is a lot of extra weight.
And will it then still be self bailing with people on board?
I would not worry about the self bailing aspect, however I would make sure that it has positive buoyancy tanks built in, even if it is flooded.

Edit - I looked back at your earlier posts, and it sounds like you leave the dinghy at a dock when not in use (?) - but if you had a very lightweight dinghy then you could easily lift it out of the water and store it ashore upside down?
I built myself a wee 2 part nesting pram dinghy 25 odd years ago - it was 7'6" long (so I could use single sheets of plywood) and it was based on Danny Greene's longer 10'4" Chameleon design - I scaled his design down a bit. I was just using the basic plan shown in the Duckworks link below.

Chameleon

Here are a couple of other Chameleon links

<span style="font-style:italic;">Chameleon</span>

Until a few weeks ago, I’d always thought that a nesting dinghy was a matter of necessity, but had little else to commend it, all practicali...

dory-man.blogspot.com

Building Robbie

www.sailorgirl.com

 

[tpenfield]

I am just wondering why you have this requirement for it to be self bailing?
I can see how it could be if you have a bilge and an extra floor higher up, but this extra floor is a lot of extra weight.
And will it then still be self bailing with people on board?
I would not worry about the self bailing aspect, however I would make sure that it has positive buoyancy tanks built in, even if it is flooded.

You must know by now . . . I want what I want.

Yes, the dinghy stays at the 'dingy dock' among a plethora of other dinghies. A pick-up and dump is not good option, since it will have a motor attached.

With a foam core structure, having a few extra inches of foam on the floor should not be an issue in terms of weight. My current dinghy does not completely self-bail because of the motor's weight, but it self bails underway at moderate speed. I plan to add 2" of additional foam to the floor, so it will self bail even with the motor.

 

[gonzo]

You must know by now . . . I want what I want.

Certainly, we all do. However, you need to decide what drives the design. If it is self bailing, stability and light weight are secondary or less. If you want light weight, self bailing is not an option.

 

[comfisherman]

Nearing week one slowly crawling up new england. Can see why the need for self bailing. Coming from the west coast we have a lot of slip docks, having all manner of coves with bouy balls and a plethora of dingys is a new experience. Seems like its been dominated by Livingston style multi hulls, and a ton of little glass rowing style boats. Although you can go 10 miles down the road and half the boats will all be little aluminun boats, place we were today was an even mix of small glass multis, aluminum dingys and a few inflatables. Seems like the lobster man use more hard side dingys and the pleasure boats have more rib dingys.

Saw more than a couple that were already half swamped before the north easter. Still having a hard time getting over the shear volume of coves and boats and coastal dwellings.

Onward to NS and Nl...

 

[tpenfield]

Perhaps it is a Northeast USA thing, but . . . The dinghy dock gets to be really crowded, particularly in harbors with lots of moorings, instead of slips. Here is a picture of the dinghy dock towards the end of the season (a few years back) . . . usually there are twice as many boats jammed in, going about 3-deep. The inflatables are the worst, since they take up a lot of space.
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Some of them are nearly full of water, as they have not been bailed out in a while. I'm sure with the Nor'easter we just got many are completely full of water. I believe my boat is one of the few that are self-bailing. So, when I leave the boat, I just pull the plug and rain water drains due to the raised floor

Here is a recent picture of my dinghy with the motor on it . . . there is about an inch or so of standing water because of the weight of the motor, so with the plug out (red string at the floor), it only self-bails to that level. I'll add 2" more floor height above the hull bottom to get it to fully drain while tied up at the dinghy dock.
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As I mentioned earlier in this thread, my goal is to make a new one with a more shaped bow to better handle the slight chop that we get in the harbor, rather than the pram-style bow. This dinghy has a freeboard height of 20", so I can stand in a corner of the boat and it does not go under. The new one will be 21" freeboard.

I'll also make storage compartments in the seats, so the fuel tank takes up less space.

 

[tpenfield]

Certainly, we all do. However, you need to decide what drives the design. If it is self bailing, stability and light weight are secondary or less. If you want light weight, self bailing is not an option.

I think a reasonable balance of those things, but in priority . . . Self-bailing #1, Stability #2, Light weight #3. I did a reasonable job in accomplishing those things on the first go-round. with the heavy OB (75 lbs, ) it is not as self bailing as it was with just oars.

From a hull design perspective, I'm going to try to come up with something (a shape) that takes the waves better and does not take-away much from the stability.

 

[tpenfield]

I stopped by the town dock this AM to check out the dinghy situation after the storm . . .

Lots of dinghies plenty full of water and nearly under water. Some have been pumped out already, others not.
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Maybe if I make a mold for a self bailing dinghy, I could have a few customers . So self-bailing is rightfully top of the priority list.

 

[comfisherman]

Oh yeah. Saw that farther north, almost everything had a couple inches of water in it. Last harbor dingy dock was chocks full of 12' carolina skiffs with bigger outboards. Definitely can see the need for self bailing if your leaving it on the morring a while.

The one in the pic with the orange bottle bailer has a seat arraignment I'm going to mock up and give a try.

Going to have to sit down and pencil out the added weight of self bailing, would be a nice feature.

 

[ondarvr]

If you like 90% of what you have now, I'd just extend the bow as you described, use a thick core on the floor, then either no core or a very thin core on the sides. The top edge could either be rolled or use a core to create a stiffer hull side. The actual hull side doesn't need a core, or be very thick to survive just fine. But if building it with a core makes it easier it won't be a problem, just a little more weight and cost.

The multi hull Livingstons mentioned earlier are one matt and one roving thick on 90% of the build and hold up OK. They are used in a similar way, but are kept on the stern of the main boat or on the beach, they are only left on the water when being used. They can be made self bailing rather easily but it's not done that often.

 

[rnlock]

I designed and built a dinghy for a friend that weighed about 50 lbs, light enough so that he could put it on top of his van by himself. It was small, but could have been somewhat larger if I'd used more expensive plywood. It was narrow enough to fit between the railings on the ramp at Marblehead.

Just how far do you usually go in your dinghy? Phil Bolger has written that the way most people use outboard motors is like taking an airplane to go to the supermarket. If you design the boat for rowing, using oars for short distances can be very pleasant. By short, depending on your inclinations, maybe half a mile or, for me, more. Some boats will row with no more effort than walking uses, and at the same speed, with a passenger.

If you MUST have that outboard, the boat will probably plane better if the bottom at the transom comes to a sharp corner.

I was thinking that a bottom of two thin sheets of marine plywood with a foam core ought to be lighter than foam cored glass. You might even use something like Highload 60 Styrofoam, which is much cheaper than most fancy foams. Plus you can hot wire cut it. BTW, I don't know if it's reliable enough, or if it's suited for really light boats, but someone sells a bilge pump that is powered by tugs on mooring or docking lines. Assuming there are waves or wind where you leave your boat. It's very simple.

 

[tpenfield]

I'm not all that worried about the weight, as long as it is reasonable (100-125 lbs would be fine). I have a rough idea of what I want to bow to look like. I think I will frame it up and shape it as I build the foam core.

This will probably be my winter project, along with restoring my sailboat.

 

[HelmutSheina]

I had a couple of those blueband 7.5/9.8s and they were great except for impeller vanes falling off and getting lodged somewhere inconvenient. You have done well to have still working 50 years after manufacture,

 

[tpenfield]

I had a couple of those blueband 7.5/9.8s and they were great except for impeller vanes falling off and getting lodged somewhere inconvenient. You have done well to have still working 50 years after manufacture,

Yes, I had a red-banded (1969) Mercury 7.5 for about 15 years, but the salt water finally did it in. I found a mildly used 1972 7.5 hp for $175 to replace it. On the 1969, I had swapped out the original LU/driveshaft for one from a 1980 motor (then a stainless steel shaft). I brought the shaft/LU over to the 1972.

Not sure if you remember, but in the 1971-72 era Mercury was migrating to a CD type of ignition, and their interim step was known as the 'Phase-Maker' ignition system (a complete piece of crap). Although I could get spark, it was not reliable at all . . . and not for lack of effort on my part. I took the magneto ignition from the 1969 model and installed it on the 1972, scrapping the Phase-Maker. The engine works pretty well.

My next stop on the Mercury food-chain may be a 1980's version . I do have an Evinrude 6HP as a back-up, and a Johnson 4.5HP long shaft for the sailboat. I tend to look for vintage motors in the lower HP range, because they had twin cylinders vs. what you see today. IMO, a single cylinder boat motor is not worth having. I had a friend's 1992 3HP Yamaha for a while . . . single cylinder . . .it never ran well. I gave it back a couple of years ago.