Originally Posted by Pete7
Matt, those figures are about right, but I think you're asking to much from a little rib and small engine. We used to dive off a Humber Alpha (4.9m) of similar size with a 55 hp mariner. Cruising speed with 4 divers and kit was 20 knots and about 26 knots empty. However other than the engine and two 5 gallon tanks of fuel there was very little furniture in the boat. Even the steering console was a simple stand up light weight affair.
If you want to use it for diving you are going to have to ditch the canopy frame and seats. That engine lift isn't doing anything for you either. Manual engines normally have a gas assist strut to help lift them up, why doesn't it work? I would have thought as a diver you could lift a 50 hp up no problem. Bottom line is you need to strip the boat of anything that isn't essential including seats, canopy frame and large amounts of petrol. Then move the stand up steering console further forward to create deck space for diving and keep the weight forward. All this will save the weight and give room for the dive kit and getting kitted up.
Agree, there's no way a 50hp is going to push all that crap at 40knts. Even 30 would be a huge surprise to me. I have a 15.5ft Ali RIB with a fairly flat deadrise (compared to some - its 15deg). It's lighter than a GRP boat at about 480lbs. Even with that flatness and lighter weight helping my speed, my 60hp Yamaha "only" gets me to 30knots empty on flat water. 4 divers aboard I max out around 26 knts.
I would guess you real max (empty) speed with that 50hp is more like 28knts:
The T-top is killing 3 to 4 off that.
The engine bracket maybe 2.
The water inside 3 or 4 more.
Net Max is hence 20ish.
You either need to strip off the superfluous gear, or get a bigger engine if you have your heart set on speed.