Engine size required for boat size?
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Category: General Forums
Forum Name: The Boat Shed
Forum Description: Discuss all things boating.
URL: https://www.fishing.net.nz/forum/forum_posts.asp?TID=121698
Printed Date: 31 May 2026 at 6:18am
Topic: Engine size required for boat size?
Posted By: Munsterdog
Subject: Engine size required for boat size?
Date Posted: 20 Oct 2016 at 7:25am
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Hi all, Somewhat related to my last message around boat size vs chop. My current rig is a 5m glass boat with a 2000 130hp. Looking for a 5.5m boat but 80℅ of them have around 115hp. Doesn't seem enough for sea conditions let alone skiing? That said they are newer engines by a good 5 years...... Anyone out there successfully ski /wakeboard on a 115? |
Replies:
Posted By: MikeAqua
Date Posted: 20 Oct 2016 at 9:39am
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115HP intuitively seems light to me. I often look at boat/engine packages and think the engines look too small. Personally I like to go toward the upper end of the recommended HP range. |
Posted By: Kevin.S
Date Posted: 20 Oct 2016 at 10:26am
| I think saying X horsepower for Y length is much to simplistic. Much depends on the hull design, and other factors. The other thing I have noticed over the years, when asking for advice on boats, is that most people seem to vastly overestimate the hp required. Compare the size of the motor to as many other boats with the same hull as you can. Then take it for a sea trial to see how it handles (preferably in fairly poor conditions). |
Posted By: Steps
Date Posted: 21 Oct 2016 at 8:10am
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As boats get bigger they proportionally , within reason get heavier...also tend to have bit more gear, under floor tanks and bit in pieces. and extra person. Also we are talking plaining hulls...not displacement, where length is an important factor ... Plaining hull, once up on the plain tends to it on a small area of the rear 1/2 of the hull...sometimes they have a flat bottom, others a curved V and a few other differences, but basically all modern trailer boats have a very similar performance on the plain...crouches formula constant between 195 and 205...and that constant increasing slightly as speed increases. Most 5.5m cabin boats.. from 80s sea nyph commander to modern rayglass/ buccaneers, and even modern ally (now have heavier plate) come in around the 1200 to 1350 kg fully loaded... for good economy thru to good performance, cruise speed etc a 115 min power thru to a 140, with 150 being a little over powered. Well propped a cruise 3800 to 4K 32 to 37 mph 5m boats one is looking at around 1000 to 1250 kg... full gross weight on the water...so looking at 90 to 115 hp. If wish to go larger 6m getting up to the ton and over (jump to things like 4 ppl comfortable, 110L tanks to 120/ 150L, bigger live bait tanks etc so very min 175 hp to 200 hp. Well propped would have a cruise around 3800 to 4000 @ 28 to 32 mph There are also significant differences in how the same hull performs and rides in chop , wakes, rough water and bars , and on flat water...between a min powered and well powered. Which also gives impressions that certain hulls 'ride better' beyond how they actually have. So yes there is a ball park for good reason to relate length to hp range. Looking for a 5.5m boat but 80℅ of them have around 115hp. Doesn't seem enough for sea conditions let alone skiing? yes you are correct, and thats why the previous owner of our boat sold it and got a newer better powered rig.. the wake/ sking for family/ friends. Suggestion: you have a boat that is top/ maybe little over powered, and good engine (???). So that engine would sit well on the back of a 5.5m. why not look around for a good 5.5m boat, thats under or min powered (90 to 115 hp ) ? Swap out the engines and flick the old boat and 115 hp engine. It may take 6 or 12 months to find a suitable boat thats priced fairly to the market to sell... They dont last long on the market, usually 2 to 4 weeks at most. The others.. like outboards tend to be on the market for 6 to 36 months We have just done the reverse ... got a bigger engine at top end of fair market price and sold off the old one at middle fair market price.. net just over couple K for the upgrade. |
Posted By: Kevin.S
Date Posted: 21 Oct 2016 at 2:34pm
| I'm sure that's great in theory, but take a look on Trademe and see how many 6m boats have 200hp+, the answer is not very many at all. I had a Viscount for a while, which is nearly 2t of 6m boat and those go very well with 150 on the back and I've seen them with less. My current boat is just under 6m and goes fine with the 115hp I have on it, and I've been out in one that had a 90 and was still fine. Given your "very minimum" of 175-200 you would think that it would hardly be able to move with so little power. I've been on a 6m boat that had a 60hp outboard, and performed well with 5 of us on board -it just depends on the boat design. |
Posted By: MikeAqua
Date Posted: 21 Oct 2016 at 4:27pm
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My 6m (heavyish) alloy boat goes well with 2 x 70HP. Offensive amount of hole shot, easy 25 knot cruise and ~35 knot top end. |
Posted By: Steps
Date Posted: 21 Oct 2016 at 4:32pm
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I'm sure that's great in theory, but take a look on Trademe and see how
many 6m boats have 200hp+, the answer is not very many at all. Theory??? No actual boats and comparing same boats with different power... REAL WORLD 2t of 6m boat and those go very well with 150 on the back And if weight up a modern boat 6m fully loaded.. gear ppl, fuel etc you will find 2T is still ball park. Go well well ... ok that is very much what one expects from the boat.... tell me what sort of economy did it have ... over all? ie nm/ L What cruise speed at what rpms? and WoT? And if weight up a modern boat 6m fully loaded.. gear ppl, fuel etc you will find 2T is still ball park. Basic physics will have a 2T boat WoT well propped (very low pitch) doing a max of 28/ 29 mph And with most outboards around the 100 hp having approx 2:1 gearbox thats giving a cruise around the 23/24 mph @ 4000 rpms A 6m buccaneer (635) loaded including dive gear.. 200 hp will cruise very nice around the 35 mph under 4000 rpms.. economically.. far better than an under powered / min powerted boat. The question as to why so many boats are under powered is because of sale technics and customer budgets and for sea trials throwing a prop with 1" or so more pitch. Customers tend to want a boat as big as possible then when comes to engine, go min... A issue many in these forums also have... lack of economy and cruise speed Same goes when re powering an old boat.. is it worth dropping a 2nd hand 10K engine on the back of a older boat that will not sell for much or, or even less ? How common is this?... well even you point out the number of low powered boats on trade me. Boat design.. well covered that above... actually look at planing hulls.. not the bow.. from 1/2 way back....they are all so similar...add a keel, few ribs or a little roundness or slight V... but bottom line there is very little difference in the hulls to make any significant difference between hull contact and the water when on the plain. Differences are stability at anchor, and bow shapes deep V to cut thru chop, or arrow to have less wind resistance on a lake and how the chines deflect water in different conditions to make a dry (or wet) boat... none of which effect the plaining parts of the hull....unless min or under powered and these parts are still in contact with the water |
Posted By: MacSkipper
Date Posted: 21 Oct 2016 at 4:59pm
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X2 Steps though if happy with a small motor fine but I have seen and experienced difference of more power on a boat from min to acceptable and it is amazing. ------------- Good fishing trip nothing breaks, great trip catch fish. |
Posted By: SpearPoint
Date Posted: 24 Oct 2016 at 4:07pm
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Munsterdog - How important is skiing to you? If you're like me and it's a main consideration I'd want it to be at the top of the recommended HP. My father and I once agreed to buy a boat on the condition that we could deep water start behind it with no dramas. So we took it for a test run, had a great day skiing and then paid for it back at the ramp. If you're worried about it, take one for a test drive.
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Posted By: RaggedJoe
Date Posted: 25 Oct 2016 at 4:46pm
| For what its worth, my Buccaneer 565 (550 by a newer name) has a 150 4 stoke. Its overpowered. WOT is verging on dangerous, and it is a heavy motor. But it pulls me up on a single ski (no easy task) and for most fishing trips is not even trying, so relatively economical. If I was buying new, I'd put a 130 on and am confident that would do the job. 115 on 5.5m is okay for skiing if you are young fit and light. I am none of those. |
Posted By: Schampy
Date Posted: 25 Oct 2016 at 5:38pm
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Ski boats have to be powered to pull some fat bugger up on a single ski on a lake in glass smooth conditions.... its obvious they need more grunt. Dont see many out at the mokes. 5.5 with a 115...... perfect for fishing and good cruising economy etc. If its a 2 stroke like yammy sws , evinrude, merc....you should be ok..person on 2 ski,s they flippin haul. Dunno if the same could be said about the early generation 4 strokes.... they seem very heavy and pretty gutless compared to todays, lighter more instant-power offerings. |
Posted By: Bounty Hunter
Date Posted: 25 Oct 2016 at 8:39pm
we also had a buccaneer 565 esprite - but with a 200hpdi - which i thought was fine - certainly it was nice to have the effortless performance - ive never been disappointed with anything that was overpowered... its really just perspective and whats 'acceptable' to yourself weight is the main determinant of horsepower requirements in planing hulls - followed by hull shape, wetted surface etc there are many 5.5m boats or thereabouts with 115hp motors on the back and they will been fine as allrounders - if skiing, and 'getting out of hole' is important to you - put a slightly shorter prop on it ------------- No disintegrations! |
RaggedJoe wrote:Posted By: Steps
Date Posted: 26 Oct 2016 at 7:59am
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if skiing, and 'getting out of hole' is important to you - put a slightly shorter prop on it Yep most have 2 props one for ski the other for general use. Ball park for a 5.5 1300/ 1400 gross weight just over min powered 115 hp would be a 15 or 16 pitch for ski and around 18/ 19 general use. |
Posted By: CoastalStan
Date Posted: 26 Oct 2016 at 5:22pm
| My 6m aluminium boat has a 115hp motor, it cruises easily at 4000rpm doing 20-22 knots. This is a safe speed for the usual sea condition I fish in. It does a max of 30 knots if I open it up. At cruise it uses around 18 litres per hour. My boat is heavier than most but it's not a hardtop so not as heavy as some. I cross a bar every time I go out and it has more than enough to keep me out of trouble. It is possible to go up to a 150hp with my boat but it is perfectly balanced as it is and it's well within my speed, comfort and economy expectations. Seriously a 200hp would just be stupid and totally uncontrollable in anything but flat water. I would rate my hull (Its a White Pointer) as one of the best sea boats for its size and it's certainly not underpowered. |
Posted By: CoastalStan
Date Posted: 27 Oct 2016 at 6:02pm
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Posted By: Steps
Date Posted: 29 Oct 2016 at 9:18am
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My 6m aluminium boat has a 115hp motor, it cruises easily at 4000rpm
doing 20-22 knots. This is a safe speed for the usual sea condition I
fish in. It does a max of 30 knots if I open it up Rem my question above? expectations one expects from the boat? Other who say travel out to channel on a regular basis .. day trip... around 6m boat will cruise around the 32 35 mph with a well propped boat... different expectations. On those numbers gross boat weight would be about 1200/ 1300 kg and if a a V4 type, merc/ johnson/ evenrude with around a 1.86:1 gear ratio and WoT in the 5000 to 5500 mark around a 15 or 14 pitch prop... At cruise you would be using around 60 hp of approx 75/ 80 available at the prop at that rpm... which still gives you a good 20% in reserve.... Basically well set up to meet YOUR EXPECTATIONS of 21 knotts (about 24 mph cruise) Assuming the numbers (Weight etc) are correct..drop a 150 hp on a and WoT goes to about 40 mph cruise @ 4K about 30 mph... about the ball park as a slightly over powered 5.5m fully loaded gross weight cabin boat |