Reply With QuoteSounds like it would be grossly underpowered - ie you would want 90 hp at least. What hp is on it now? How it performs now would be a clue. It would help if you gave the make and model of the boat as well.Originally Posted by trasten
Hi,
I'm new to boating and just got a 5.5m half cabin with a loud, 2 stroke, really old engine. Looking at a the 4 stroke Honda BF40 to replace.
Is this a reliable motor? Any issue I need to be aware of when inspecting the motor?
Would it be grossly underpowered for my hull?
Looking to mostly cruise on the Swan river in Perth, perhaps on the very rare occasion venture close out to sea but not far.
Thank you!
Sounds like it would be grossly underpowered - ie you would want 90 hp at least. What hp is on it now? How it performs now would be a clue. It would help if you gave the make and model of the boat as well.
It has a 80hp motor at the moment. The rego says it was made by Chivers, a WA company.
I found these videos:
I don't have to go nearly as fast as that guy.
Yes but bear in mind that it is an aluminium boat which will be lighter and probably have a shallower deadrise. It also appears to be completely unladen. You would probably be better off with your existing motor if it is running OK.
Following the thread below suggests a much lower power as backup eg 4hp. Would have thought 40hp would be ok for river use? Thank you.
http://www.ausfish.com.au/vforum/sho...kup-motor-tips
for a planning hull you need enough hp to break the suction affect and get up on top of the water, you then reduce power to hold it there
for a sub planning hull you can get away with a lot less hp but also travel at a lot slower speed
how do you reconcile dropping 50% of the current hp and consider it a good outcome ?
that suggestion of a back up engine is just to get you home and not try to beg for or pay for a tow home
not familiar with the hull..except for a quick peek online at a chivers 5.5m ally boat for sale...actually a few...they mainly seemed to have bigger motors than even 80hp....so Im thinking you might need to speak to the manufacturer first, perhaps some other Chivers owners...on first impression 40 hp is below minimum.Put a couple of people and some gear in the boat as well and it could have that engine working very hard.....
Grossly underpowered. I had. 40hp on a 4.5M ally runabout and it was barely enough. Put 3 blokes in plus a bit of gear and it really struggled to get on the plane.
The americans have a horse power calculation for certain size boats it gets quiet technical due to single, mono, glass, plastic, ally, composite hulls
But a quick way to look at it is length of boat in feet times boats width in feet = recommended hp for a single fibreglass hull
For an example my hull is18.7 foot x 7 foot = 130.9hp
Now if you own a haines hunter all that calculation goesout the window and u lob a v6 200hp on itlike they all do..
Heres the formular it links back to ausfish
Hi all,
Thanks for the comments but most are implying I'm trying to get to a good speed. 95% of the time I'll be boating in the swan river in perth where a lot of the speed limit is 8 knots and even 5 knots further upstream. The remaining 5% is possibly ocean fishing not far off from the shore and I'm happy to not do this at all if a new motor is going to cost thousands extra.
If the motor can deliver ~15 knots and or a bit slower if against current and wind without struggling too much I think that would be more than enough for what I need.
Just browsing around to get data for my purpose and found these:
25 ft sailboat with 13hp diesel gave 6.5 knots max: https://www.quora.com/What-size-of-m...a-17-foot-boat
18.6ft half cabin with 135 hp gave 38 knots. One third the power should give well above one third the speed due to the drag equation : https://www.hitechmarine.com.au/chiv...0l-tks-engine/
I saw a boat yesterday with very similar length and shape to mine but seemed to be powered by quite a smaller suzuki motor compared to mine. Don't know enough to determine model.
Any other thoughts?
Cheers.
Running round at that speed your constantly in the hump of trying to climb onto the plane
you will be labouring the engine while doing this and also doing it with a low capacity engine
I would not be taking any planning hull out into a high wave or current area with out the recommended minimum hp to make the boat plane
I think you need to research what engine damage comes from labouring a engine in a underpowered hull
you need to decide what type of boat you want first as it’s all fanciful ramblings till then
Agree totally and comparing a 5.5m half cabin to sailboats and mercruise drives kind of indicates a boatie virgin as the concepts between what he has and the others is like chalk and cheese and has absolutely no relevance to this discussion..
Trasten, go with the minimum for the hull, I think that 65hp would be the absolute minimum, as mentioned you would be killing the 40 hp. The other thing to consider is resale value, with a 40hp on it you would need to drop th3e price considerably. I had 14'6" F/G Half cabin put an 85 hp on it (I had considered a 65hp) which at the time I thought was too big but at the end was just right. At the end of the day when you do need the power you will appreciate it.
Two things. First - if you go outside you need power. Limiting yourself to 15kph in a boat that size is dangerous beyond belief. Second - if you come onto a Forum of people who are absolute gurus in things like this, stop making arguments to disregard the advice. Listen to the experts, who have been there done that many times, or follow your own advice and make a major mistake.
As Dignity and others have said displacement and planing hulls are two different types of hulls and each has different power requirements, A yacht is generally a displacement hull that pushes its way through the water slowly, generally below 10 knots economically and easily, unless its a hybrid/ semi displacement/racing yacht ( they have different shaped hulls and bottoms), so it doesn't need a lot of power to reach that 10 knot speed, speed boats on the other hand are planing hulls and need to push their way out of and then stay on top of the water and get to much faster speeds, so need to overcome the resistance of climbing out of the original hole in the water and then sit and stay on top on top, so much more power is required.
Depending on the the type of boat, 15knots may be borderline between displacement and planing speed so if it is a displacement hull such as a yacht you would be pushing the motor flat out trying to get the yacht out of the water when it wasn't designed for it. Doing the same with a planing boat and a small motor, it may not have enough power to get out of the hole and up on the plane and stay there, add current, oncoming waves and wind and you may be fighting a loosing battle. The flatter the boats bottom the easier it may be plane, so just because you can put a 40HP on a larger boat does not mean that it is suitable or advisable.
So while it may be satisfactory in the river at 5-8 knots it won't be in the open ocean! There is a reason that they have minimum power requirements stated on hulls. I tend to go for the upper end of the requirements.
Just watched the video of the 40HP Honda, it was on a ally boat which no doubt is considerably lighter than a fiberglass boat and the test was done on relatively flat waters, so to compare between yours and the one in the video you would also have to compare the hull shape and weight. I don't know what type you have but as I said before, just cause someone does it and puts up a video of it does not mean it is a good idea! As the saying goes ... your boat and your life but make sure it is only your life at risk.
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