Leicester Fisheagle Posted July 23, 2006 Report Share Posted July 23, 2006 Can anyone help me with a little problem? I have just returned to boat fishing after 12 years. I have a small 16ft boat with a max design speed of just over 18kph. It is fitted with a 15hp 2 cylinder 4 stroke Yamaha engine (1988/99) which is the max size recommended for the boat. Given average conditions and load etc. I can easily work out my boat's speed using my GPS. No real problem so far. What I would like to find out is 1) What is the max. rev range for the above engine? 2)What revs is it doing at its max design speed of around 18 kph? I have an old Tester/Tune up Analyzer which amongst other things has a Tachometer for 4,6 and eight cylinder engines. 3) Given that 4 cylinders is the lowest setting on my tester, assuming I can find the right wires, I believe that 2 cylinders will give me a reading twice what the tester shows on a 4cylinder setting. 4) Which wires does it need connecting to? When I used to do cars and bikes in the 60's they all had coils and the connections were earth (black meter wire) and live (red meter wire) to the non battery connection on the coil on neg earth vehicles. The reverse applied on the positive earth vehicles. 5) I do not have a wiring diagram and have looked for one on the Internet. Any help apart from buy a bigger boat and engine with a rev counter would be much appreciated. Quote Link to comment Share on other sites More sharing options...
Afishionado Posted July 23, 2006 Report Share Posted July 23, 2006 Blimey! With the greatest of respect do you feel that all of the information you have asked for is of any real use? Max engine rev's Mmm probably 5500, ideal eco cruising 3500, with a suitable flatish hull but entierly different with a full bodied displacement hull. But nothing is going to change or matter with all the knowledge you ask for. As the performance of the boats hull and the engines required torque at whatever hull displacement speed/water condition/ engine rev's/propeller pitch x diameter is what it will be. Let your ear be the guide, if the engine is over stretched you will hear it, a rev counter on a 15hp/16ft boat is realy un-necessary, as the hull shape and displacement has far far more affect on things than your engine. By that I mean that a 15 hp engine will gain the hulls maximum dispacement speed as will a 4hp engine too. One could go on pouring power into the 15hp's output but the boat will not go any faster, it will just sit down at the back and raise it's nose. To be of any real help in answering What is the max. rev range for the above engine? 2)What revs is it doing at its max design speed of around 18 kph? One would have to know what was the hull type it was coupled too. Mad Mike Quote Link to comment Share on other sites More sharing options...
Leicester Fisheagle Posted July 23, 2006 Author Report Share Posted July 23, 2006 (edited) Thanks for the information. Just interested to try and find out what revs it was doing at max design speed. Given it is a displacement hull didn't want to waste extra power causing the problems you mention. Edited July 23, 2006 by Leicester Fisheagle Quote Link to comment Share on other sites More sharing options...
Afishionado Posted July 23, 2006 Report Share Posted July 23, 2006 If it is a displacement hull then the max speed can not possibly be 18kph (or what ever that is in Knots. There is a quite strict equasion of calculating the maximum hull speed of a displacement boat. Which is Sq root of WL X 1.5 (to 1.7 if one is talking about a very clean narrow and long hull) equals the hull speed in Knots. For most boats the Sq root of the waterline length X 1.5 = hull speed. If your boat had a waterline length of 16ft the squ root is 4. Times 1.5 = 6knts and most importantly it will always be in the region of 6knts no matter what power is applied. All that more HP will do is shift more water from under the stearn which created a hole that the hull can not climb out of. So....... If you engine has a nice economical run speed of say 3300 revs (which is probably about right) but can not attain those rev's (by ear will do) then the engine is 'over' prop'ed. The pitch is to coarse and the dia too large. If on the other hand (very unlikely) it rev's it's little head off and the boat hardly accelerates and eventualy only goes along at a slow walk, then it is 'under' prop-ed. Propeler's can be re-pitched to a max' of 1" either way. Mad Mike Quote Link to comment Share on other sites More sharing options...
Pugwash Posted July 23, 2006 Report Share Posted July 23, 2006 Gulp!!!! Quote Link to comment Share on other sites More sharing options...
Leicester Fisheagle Posted July 23, 2006 Author Report Share Posted July 23, 2006 Double Gulp!!! Thought it was simple question but obviously not. Will try and work through what you have kindly taken the trouble to write. My wife used to teach physics and maths so will be calling on her help. Quote Link to comment Share on other sites More sharing options...
Maverick Martin Posted July 23, 2006 Report Share Posted July 23, 2006 Allan If your boat has a design speed on 18kph it isn't a displacement hull. Which make and model of boat do you have? Most outboard motors will have an optimum rev range which is usually somewhere between 5000 and 6000 rpm any Yamaha marine engineer will be able to give you the rev range for the specified model. Alluminium props can be tweaked by quiet a lot and as a guide eack deg of pitch equals around 200 rpm. Stainless steel props can only be tweaked a small amount Would agree with Mike that you shouldnt need to go to the extreme of having rev counters cause if it sounds right it probably is. Martin Quote Link to comment Share on other sites More sharing options...
Brian Posted July 24, 2006 Report Share Posted July 24, 2006 18kph = 9.7 knots, must be a semi-displacement hull Quote Link to comment Share on other sites More sharing options...
Afishionado Posted July 24, 2006 Report Share Posted July 24, 2006 If indeed the 'k' is a kilometer and not a kurnot This busness of hulls being rated to X knots is a purely arbitrary thing placed on a hull by a manufacturer or again an arbitrary speed limit sugested by some insurance underwriter at which the insurance premiums rise. Most often these stated figures are the whim of a retailer whishing to impress a potential buyer with an 'informative' specification. In this country there is no public authority that tests the sea worthyness, durability, useability, or safety of small craft (more's the pity). The questions of engine output on boats is greatly misunderstood. for example quoted HP. Well is that HP at the crank? The gear box? The propeller? Which is worst an under-proped engine reving at a max rev rating of 5000rpm and going nowhere? Or an over prop'ed engine strugling to get up to 1500rpm and straining it's little ball cocks off. To get the correct answer as to what engine HP/useable rev range/economy/efficiency is required one must also answer the question as to what craft it is to used on and the likely use that the craft will undertake. Mad Mike Quote Link to comment Share on other sites More sharing options...
Brian Posted July 24, 2006 Report Share Posted July 24, 2006 I was just going by the fact that he stated the max design speed of the hull was 18kph, therefore it can't be a displacement hull. Think about it, kph must be kilometers per hour. Knot is a unit of speed in it's own right without "per hour". Therefore if the "k" in kph was knot, it would = Nautical miles per hour per hour. (which is acceleration, pretty poor acceleration at that). Quote Link to comment Share on other sites More sharing options...
Afishionado Posted July 24, 2006 Report Share Posted July 24, 2006 It may have been a mistake of the writer is the point I was making. I have never come across a boat with a stated speed in anything but Knots although I imagine there will be someone to tell me that in Ulan Bator that's what they do and also they measure in Yak power. Mad Mike Quote Link to comment Share on other sites More sharing options...
Brian Posted July 24, 2006 Report Share Posted July 24, 2006 Mike I agree, I've never seen it stated in anything other than knots either, but not seeing the manufacturer's paper that he refers to, I gave him the benefit of the doubt. Regardless of the units, even if it's 18 Yaks, it can't be a displacement hull, unless of course a Yak equates to 0.33knots!!! Over. Quote Link to comment Share on other sites More sharing options...
TomBettle Posted July 24, 2006 Report Share Posted July 24, 2006 To put you all out of your misery and only becasue Alan only pops on te forum from time to time. The boat he has is an Orkney Longliner. I would have to agree that stating performance in KPH would be most odd. owever MPH is often used by American manufacturers. Quote Link to comment Share on other sites More sharing options...
Afishionado Posted July 24, 2006 Report Share Posted July 24, 2006 To put you all out of your misery and only becasue Alan only pops on te forum from time to time. The boat he has is an Orkney Longliner. I would have to agree that stating performance in KPH would be most odd. owever MPH is often used by American manufacturers. Ah in that case I don't think the 15hp will have quite enough power to put it on the plane, therefore the rules regarding displacement boat speed being governed by the W/L length apply. Except that he might get about 8knts out of it before the 'hump' became too big for the boat to climb over and plane. A 25 hp engine will make the world of a difference to the top speed. Mad Mike Quote Link to comment Share on other sites More sharing options...
Brian Posted July 24, 2006 Report Share Posted July 24, 2006 To put you all out of your misery and only becasue Alan only pops on te forum from time to time. The boat he has is an Orkney Longliner. I beleive the Longliner has a design speed of 7 knots. Quote Link to comment Share on other sites More sharing options...
Afishionado Posted July 24, 2006 Report Share Posted July 24, 2006 If that is so, QED. Mad Mike Quote Link to comment Share on other sites More sharing options...
Maverick Martin Posted July 24, 2006 Report Share Posted July 24, 2006 To put you all out of your misery and only becasue Alan only pops on te forum from time to time. The boat he has is an Orkney Longliner. I beleive the Longliner has a design speed of 7 knots. I would agree with a maximum speed in the region of 7 knots as being about right. Allan your 15hp engine will achieve the boats max speed with ease. Be aware of your engine noise and the attitude of the boat. As soon as you feel the boats rear dig in then back off the throttle a little, this will be your optimum cruising speed and as Mike has stated will be around 5/6 knots Martin Quote Link to comment Share on other sites More sharing options...
Afishionado Posted July 24, 2006 Report Share Posted July 24, 2006 Quote......As soon as you feel the boats rear dig in At this point you use the same term as the Royal family do in such circumstances "MY ARSE!" If you have a passenger with you getting them to sit foward of amidships will help the craft stay level and actualy the boat will go a shade faster two up than with just one person sitting at the rear. Mad Mike PS we haven't even discussed the various differences between the boats performance in fresh water as opposed to salt, then of course there is atmospheric pressure to take into account and ambient temperature let alone air humidity because as everyone knows a higher humidity will increase poweroutputbyafactorof1+tothepowerod4.73recuringdevidedbythelastprimenumberxPiAghhhhhhhhhhhhhhhhhhhhhh Quote Link to comment Share on other sites More sharing options...
Leicester Fisheagle Posted July 24, 2006 Author Report Share Posted July 24, 2006 Seem to have really opened a can of worms with this. The KPH was kilometers an hour as this was the speed indicated on my GPS. sorry if this caused confusion. Obviously wind, tide etc could have played its part.The max design speed indicated by Orkney was 10 knots which makes about 18 kilometers an hour. Further research based on a call to Orkney have suggested an "optimal cruing speed" of 7/8 knots. Clearly from what everyone is saying the engine is coping easily at these speeds as none of the warning signs suggested are evident. Suspect that once I've got back into things, hopefully caught a few fish and had some fun and got to know the area better and made more contacts in the Club that Warrior I was within a few minutes of buying at Southampton Show last year or if Tom B. is reading this the elusive Janneau that the missis fancied at around Quote Link to comment Share on other sites More sharing options...
Afishionado Posted July 24, 2006 Report Share Posted July 24, 2006 Allen you just asked about an engine, but when it comes to your very reasonable expectation "hopefully caught a few fish ". Strewth mate that really will be a can of worms (sic) . For a start why not try someth simple question like "When fishing for bream how long should my trace be?" I would guess the totaly conflicting answers will run into 3 if not 4 pages, several 'bitch' fights and at least one resignation! I seem to remember a year ago or so someone asked "are rag worms a good bait?" The resultant arguments lead to one suicide, three cases of slander and 12 month course of therapy for one of the unfortunates who dared to become involved That's the great thing about fishing .... It's so bloody relaxing Mad Mike Quote Link to comment Share on other sites More sharing options...
Leicester Fisheagle Posted July 24, 2006 Author Report Share Posted July 24, 2006 Can see why Weymouth Angling Society no longer have a Web Site. Couldn't understand why they said it was too much hassle and nobody wanted to run it. Thay have obviously all joined PBSBAC!!! Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.