Graham Nash Posted October 20, 2009 Report Share Posted October 20, 2009 (edited) Hi, I have a lovely stable little Plymouth Pilot 18. It has a brand new 17hp inboard diesel engine which sips fuel and gives a maximum speed of 7 knots..... but my need for speed is getting the better of me!! I am told the maximum speed of an 18ft displacement hull will be no more than 7 or 8 knots and if this is the case i might have to consider a different boat. An increase to about 10 to 12 knots would be acceptable to me for now Is there any way i can increase the transome strength (even build an external transome) to take an outboard for those days i want to travel to far off places?? Even St Albans Head or the Needles is an all day trip for me at the moment. Help me please, I would love to keep my "little Sal" but she needs to be able to get me places quicker or she might have to go. Edited October 20, 2009 by Graham Nash Quote Link to comment Share on other sites More sharing options...
Brian Posted October 20, 2009 Report Share Posted October 20, 2009 (edited) You've been told correctly. Max speed for a full displacement hull is approx: = Y x (SQRT waterline length) Where Y = a constant, ( 1.3 to 1.5 ) = 1.5 x (SQRT of 17) = 1.5 x 4.12 = 6 knts Approx. For more speed another boat is the answer. Edited October 20, 2009 by Brian Quote Link to comment Share on other sites More sharing options...
great white Posted October 20, 2009 Report Share Posted October 20, 2009 Hi as Brian says it sounds right and looks like a nice boat with limited speed. You can gain a little speed by upping the horsepower, but not a lot and it is not usually worth the hassle and expense. the extra weight sits the hull deeper in the water, increases the displacement thus cancelling out the benifit of the extra power and burning more fuel. Looking at the picture the boat probably has a keel and rudder steering, adding an Outboard on the transom would not work. Its a change to a semi displacement or Planning boat to get the cruising speed you are after. But that will up the fuel consumption and costs per trip. Charlie Quote Link to comment Share on other sites More sharing options...
Graham Nash Posted October 20, 2009 Author Report Share Posted October 20, 2009 I will be gutted if she has to go, but thanks for the replies chaps. Tell me, What is the benefit of a displacement hull then? Are they more stable? Coz I do suffer quite a bit from sea sickness and anything less stable would possibly not get used as much due to the lack of comfort. Quote Link to comment Share on other sites More sharing options...
codpiece Posted October 20, 2009 Report Share Posted October 20, 2009 Hi, I run an Orkney long liner, same type of craft as yourself, I push it with a 15h Tohatsu, max speed 8knots, but great sea boat, good handling, and like you cheap to run!! I personaly am outside the must go quicker at all costs club (probably my age) in return for a leisurely ride in comfort with no life threatning injuries or bruising !! The Island is an all day trip for me in selected weather as well, but is it not so for everyone? As I see it if the Island is the extent of your desire you are ok, how often will you want to go further? A big consideration for me was running expenses etc, so boat on trailer, and an occasional charter trip when the urge takes me. Derek,V Quote Link to comment Share on other sites More sharing options...
great white Posted October 20, 2009 Report Share Posted October 20, 2009 Full displacement boats tend to be stable but it depends on hull design Some I have worked on roll horribly, but small ones tend not too. Some ships can be go fast even though they are definatly displacement, one of mine did over 30 knots, but it had 25,000hp per shaft very economic 4 tons of fuel per hour each shaft Back too the tread Really it is down to how far and how often compared to costs If you like to stay inshore fishing most of the time, keep the nice boat you have but take a trip on other boats or charters when the urge to go further gets you. If you want to go further every trip then a change of boat may be required. As you say you suffer at sea, that may not be the best option. Charlie Quote Link to comment Share on other sites More sharing options...
Graham Nash Posted October 20, 2009 Author Report Share Posted October 20, 2009 Derek & Charlie, what you both say makes perfect sense. The weather is rarely good enough to do the Island anyway so I suppose a trip on a charter on the odd occasion makes much more sense than losing a small fortune on my boat just to get something else that might not tick all the boxes either. I think i will stick with my lovely little boat for now and be grateful for what I have. Many thanks for the replies. Quote Link to comment Share on other sites More sharing options...
codpiece Posted October 21, 2009 Report Share Posted October 21, 2009 (edited) Charlie, Did you keep it on a trailer in your front garden?? Derek V Edited October 21, 2009 by codpiece Quote Link to comment Share on other sites More sharing options...
Sinbad Posted October 21, 2009 Report Share Posted October 21, 2009 Well the answer seems to me to be a "Cut and Shut" to increase the waterline length to arround 60ft.... At least you wont have to modify the transom.... This should give you 12knts or so. On a serious note, the suggestion to spend the odd charter trip, or go out with a club member is a much cheaper option, and keeps it economical. Regards Paul Quote Link to comment Share on other sites More sharing options...
Brian Posted October 21, 2009 Report Share Posted October 21, 2009 (edited) Well the answer seems to me to be a "Cut and Shut" to increase the waterline length to arround 60ft.... There's always a way around the problem. Nice one Paul Edited October 21, 2009 by Brian Quote Link to comment Share on other sites More sharing options...
dainichi Posted October 22, 2009 Report Share Posted October 22, 2009 Hi There In my younger days I had several different boats, Starting with a small flat bottom dinghy, I then wanted a larger boat so bought a displacment hull this wasn't fast enough so bought a fast cruiser This proved too expensive to run and was uncomfortuble in anything but calm conditions I finally gave it all up and for many years and pursued other pastimes, but the one boat I missed most was my small Dinghy. I am happy to say I now have another slighty larger small flat dinghy with a 4hp outboard, which I am able to trail and easily self launch, is extreamly economical and great fun. In but a few trips out in the summer myself and my good friend and "First Mate" Rita have caught nine species from the Poole and Christchurch harbours and just outside in the bay, Including a very nice Trigger Fish from Poole Harbour caught and returned by Rita (she ALWAYS manages to catch more than me) though I have to bait the hooks and remove the fish. I just wanted to say, Ive never been happier than I am with my small dinghy, So my advice would be stick with your displacement boat! Kind Regards Alan (an Ancient Mariner) Quote Link to comment Share on other sites More sharing options...
Graham Nash Posted October 22, 2009 Author Report Share Posted October 22, 2009 Thanks Alan. For now I have decided to stick with her. I often see sportier boats rocking like a bronco while we are sat quite comfortably on mine fishing happily, but i also quite often see those same boats arrive at the slipway as I am already setting off and they fly past me before I even get to the harbour entrance. To be honest at this moment in time I am fortunate that the cost of trips is of less concern than the comfort so I suppose for now comfort has to take precedence. Quote Link to comment Share on other sites More sharing options...
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