Dinghy Plane

Nissan 9.8HPSephina has an Avon RIB as it's tender that allows us to get ashore from anchorages. Talking to people and reading a lot told me that in the Caribbean it is your taxi, so it needs to be quick. For it to be quick it needs to be able to "plane" and this is defined as "the mode of operation for a waterborne craft in which its weight is predominantly supported by hydrodynamic lift, rather than hydrostatic lift (buoyancy)”.  In simple terms, when the forward motion is enough, the majority of the hull lifts out of the water and only the back part is touching. This reduces drag and the speed increases. The trick is to get the dinghy out of the water first and this needs a powerful engine or a light dinghy. The Avon RIBs are not light, having a ridged base with blow up tubes on the sides and front. The original engine fitted was a 8HP Yamaha. This would not quite get the dinghy on the plane with one person aboard. I therefore ordered a 20HP Honda 4 stroke motor with electric start. Bliss. It duly arrived and at 56kg was a handful to mange for the two of us but we managed to get it on the RIB and off we shot. It did get on the plane with the two of us on board but only just. We used it for the 2012 season in the UK and fitted a crane to the transom of Sephina so we could store it for the Atlantic crossing. But.... It was so heavy the bracket on the transom was not big enough to hold the weight. As there were so many jobs to do and it was fine on the back of the dinghy lifted up on the davits, it fell down our list of must do's until a week before departure. We decided to weld a bracket to the side of the davits but when James the welder looked at it he was unhappy with the loads the welds would have to support. With only two days to go we cut our losses, left the 20HP at home and went to the chandlery to get a lighter motor. All that was available was a 5HP Honda, it would never get us to plane but It would at least get us on and off the boat, was significantly lighter and would fit the existing mounting on Sephina. My mate Jon knew it was not really powerful enough but time had run out. So here we are in Grenada and now we need to sort this properly. We have had enough of pottering around in a slow dinghy and taking ages to go anywhere. Reading around and looking at what other cruisers have led us to the Nissan (Tohatsu) 9.8HP two stroke. (2 strokes are a lot lighter than 4 strokes but are banned in a lot of countries as exhaust emissions are too high). Talking to the supplier, he assured us it would get the RIB on the plane with two people aboard. We purchased it and mounted it and we set off from the dock full of anticipation. Dinghy TransomeIt was a lot faster than the 5HP but it would not plane. Ju got out and I set off to see if it would plane with one. It just about did and when the forward part of the hull was out of the water the speed increased and I was flying. More reading and the addition of “Dole fins” which add a wing just above the prop seems to help get on the plane. These were fitted and certainly made it quicker to get on the plane one up but it was still tantalising close with two. More reading and a finer pitch prop would give more initial power to get up to planing speed, this took two weeks to arrive. In the mean time we had met another couple on a fabulous Oyster 53 Amelie who were bemoaning the fact that as another boat had parked next to them they could no longer lift their dinghy onto the davits to drain it of water. I asked them if they had a simple plastic bailer which could do the job just as well! The owner looked at me quizzically and said that would not drain the space between the floor and the hull. What space between the floor and the hull I asked? He showed me a second drain hole just below the main one which drains water out of the ridged part of the hull. It is known as the inner hull drain plug. I had never seen this drain plug and when I went to look at our dinghy sure enough there was a second plug. It had never been opened and the washer had perished. I hoisted the dinghy onto the davits opened the normal drain plug to let the rain water out of the floor and the asked Ju to get on the paddle board to reach the newly discovered drain plug. She ventured forth and unscrewed it to a sudden shriek as foul smelling water shot out all over her. She bolted to the shower block not best pleased. The water continued to pour out for at least 10 minutes and was so foul our friends on Amelie thought we were draining the toilet holding tanks!  The new prop had now arrived and it was a five minute job to fit it. So with new prop and a lighter dinghy Ju and I set off to see if it would plane. It was like a bull out of a gate.  It sprang onto the plane in seconds and sped off at what seemed lightening speed. I suspect the removal of that foul smelling water was the major factor.