What HP should I go with?

[greatlakesmann]

I posted this over on The Hull Truth, but a member recommended I try asking here. So here I am!

Pretty much copy and paste:

I am looking at getting a smaller footprint inflatable for a fishing craft. I have a larger trailer boat, 19 foot fiberglass deck boat, but sometimes I want to go with a smaller craft or take a different vehicle.

I am looking at this one, a 15 foot soft bottom inflatable boat. A Saturn KaBoat, it is long and skinny. Link here: https://saturnrafts.com/15-saturn-kaboat.html.

I was thinking of getting a 5hp Mercury 4 stroke to put on it. I'm not looking to win any speed records here. I mainly want something small and light, since this boat and motor will be rolled up and put in the back of my car, and I will have to lift the motor and put it on the transom each time. Rated HP is 10hp 4s and 15hp if 2s. Website says a 5hp will push this boat at 15mph, but I'm not sure how accurate that is. Rated total capacity is 900lbs and I am much less than a third of that, adding another person and gear will take up more than half of the capacity.

The largest Merc I could go with is a 6hp, at 8lb it becomes a 2 cylinder and the weight jumps up. I would like to stick with 6hp and less. I will be using this on smaller inland lakes and rivers, the largest frequented lake being around 2,500 acres. I want something that has some power behind it I don't need to plane out, but I would like something that will keep me going forward when the wind and waves start to pick up. I considered a trolling motor, but the 2500acre lake I frequent is 4.5 miles long. When I'm on the other side across from the launch and the wind starts to blow, I want a gas motor with extra fuel.

Would I wish I had a 6hp if I go with the 5? I know, you never wish you had less HP but I'm assuming if I'm loaded down, neither one will get me beyond hull speed, which would negate the extra HP difference. At hull speed, the 5hp might work a little harder and burn more fuel, but I plan to bring extra fuel anyway. If I'm running light and can get on plane, the 6hp may get me going a little faster. The 5hp is a little cheaper, not that either of them are very costly, and it may be easier in the future to find a small boat rated for 5hp vs a 6.

Which HP would you recommend for a soft bottom inflatable, given my parameters? Light weight and able to combat moderate wind and waves. I'm not going out on the inflatable if it's nasty.

[chipko]

Hi and welcome.

I agree that you will hardly notice much difference between a 5 and 6hp motor on the water. However, since the 4, 5 and 6hp motors all share the same block and therefore weight, then if cost is not an issue, I’d personally go 6hp. Better to have a bit extra power than not, no matter how marginal that may be.

The long waterline and narrow beam of these kayak boats make them very efficient and do seem go well with the smaller motors. One up, lightly loaded then I’d expect to see the claimed planing speeds of up to 14/15mph realistically achievable with 6hp. Two up plus gear then you’ll more than likely be down to displacement speeds only, but even at a relaxed half throttle will easily keep you plodding along at 6/7mph.

For more power at the same weight then you’d need to go old school and consider something like a 9.8 two stroke.

[chipko]

Hi and welcome.

I agree that you will hardly notice much difference between a 5 and 6hp motor on the water. However, since the 4, 5 and 6hp motors all share the same block and therefore weight, then if cost is not an issue, I’d personally go 6hp. Better to have a bit extra power than not, no matter how marginal that may be.

The long waterline and narrow beam of these kayak boats make them very efficient and do seem go well with the smaller motors. One up, lightly loaded then I’d expect to see the claimed planing speeds of up to 14/15mph realistically achievable with 6hp. Two up plus gear then you’ll more than likely be down to displacement speeds only, but even at a relaxed half throttle will easily keep you plodding along at 6/7mph.

For more power at the same weight then you’d need to go old school and consider something like a 9.8 two stroke.

[greatlakesmann]

Originally Posted by chipko

Hi and welcome.

I agree that you will hardly notice much difference between a 5 and 6hp motor on the water. However, since the 4, 5 and 6hp motors all share the same block and therefore weight, then if cost is not an issue, I’d personally go 6hp. Better to have a bit extra power than not, no matter how marginal that may be.

The long waterline and narrow beam of these kayak boats make them very efficient and do seem go well with the smaller motors. One up, lightly loaded then I’d expect to see the claimed planing speeds of up to 14/15mph realistically achievable with 6hp. Two up plus gear then you’ll more than likely be down to displacement speeds only, but even at a relaxed half throttle will easily keep you plodding along at 6/7mph.

For more power at the same weight then you’d need to go old school and consider something like a 9.8 two stroke.

Thanks for the reply. My thought was that at displacement or hull speed, I'd probably see very little difference between the 5 and 6 HP, the 5 might have to work a little harder, but not by much. If I can plane out, I figure the 6hp might give a little more top end. Another factor I didn't consider, the 6hp may get me moving a little quicker, as it likely has a bit more torque as well. So it may be more responsive, which is a good thing.

Cost isn't really an issue, I'm going to save up for it. I've got other things I need to drop money on right now, this is a backseat problem. Just trying to wrap my head around all the options currently.

However, the cost is a slippery slope. I've done the math, and at my local dealer, the cost from 2.5 to 3.5hp is about 230 bucks, then from the 3.5 to 4 is another 230 or so. From the 4hp to the 5hp is another 230, and again, from the 5hp to 6hp is an increase of around 230 dollars.

So my mind naturally goes, well, why get a 2.5 when you can get 3.5 for a little more? But at 4hp, which is only a little more than the 3.5, you add on a reverse gear and an aux tank option. Idk, 4hp is kinda odd, at that point you might as well step up to the 5hp for only a few more dollars....you can see where that line of logic ends up. Next thing you know, I have a 350hp outboard hanging off the back of my Kayak Boat. Not really, but you get the point. My mind naturally works that way sometimes, so I have to put a stop to it somewhere.

I haven't decided where to stop yet, but I think the top end would be the 6hp. The cost difference going to the 8hp is about the same, however, that's when you start to see the weight jump up from 57 pounds to the high 80's, I can't remember the exact number.

Current options are the 5 and 6hp, still got some more deciding to go. Part of me says, if there's no real difference, then save a few bucks and go with the 5. In reality, the difference is 1hp. I'm probably thinking about it way too much, it's not like I'm deciding between a 115hp and a 150hp, which would make a much larger difference.


Sorry for the long rambling reply, just my mind working overtime...at work.

[Peter_C]

A 2 cylinder motor will be much smoother and quieter, vs a 1 cylinder. Often moving up to a 6hp means 2 cylinder.

[greatlakesmann]

Originally Posted by Peter_C

A 2 cylinder motor will be much smoother and quieter, vs a 1 cylinder. Often moving up to a 6hp means 2 cylinder.

On the Merc's, the 6hp is a single cylinder. The 8hp is a 2 cylinder. However, weight is an issue. The 6hp is 57lbs while the 8hp is 84lbs. It may not sound like a lot, but trying to stuff that in the back of a car or lift it on and off a transom would get tiring after a while.

[CherMac]

Originally Posted by greatlakesmann

I posted this over on The Hull Truth, but a member recommended I try asking here. So here I am!

Hope you are getting more help here than on THT. I am touched that you listened to my recommendation.


I wish my kids would be as willing to take my advice...lol

[CherMac]

Originally Posted by greatlakesmann

So my mind naturally goes, well, why get a 2.5 when you can get 3.5 for a little more? But at 4hp, which is only a little more than the 3.5, you add on a reverse gear and an aux tank option. Idk, 4hp is kinda odd, at that point you might as well step up to the 5hp for only a few more dollars....you can see where that line of logic ends up. Next thing you know, I have a 350hp outboard hanging off the back of my Kayak Boat. Not really, but you get the point. My mind naturally works that way sometimes, so I have to put a stop to it somewhere.

I haven't decided where to stop yet

[beamishken]

Originally Posted by greatlakesmann

Thanks for the reply. My thought was that at displacement or hull speed, I'd probably see very little difference between the 5 and 6 HP, the 5 might have to work a little harder, but not by much. If I can plane out, I figure the 6hp might give a little more top end. Another factor I didn't consider, the 6hp may get me moving a little quicker, as it likely has a bit more torque as well. So it may be more responsive, which is a good thing.



Cost isn't really an issue, I'm going to save up for it. I've got other things I need to drop money on right now, this is a backseat problem. Just trying to wrap my head around all the options currently.



However, the cost is a slippery slope. I've done the math, and at my local dealer, the cost from 2.5 to 3.5hp is about 230 bucks, then from the 3.5 to 4 is another 230 or so. From the 4hp to the 5hp is another 230, and again, from the 5hp to 6hp is an increase of around 230 dollars.



So my mind naturally goes, well, why get a 2.5 when you can get 3.5 for a little more? But at 4hp, which is only a little more than the 3.5, you add on a reverse gear and an aux tank option. Idk, 4hp is kinda odd, at that point you might as well step up to the 5hp for only a few more dollars....you can see where that line of logic ends up. Next thing you know, I have a 350hp outboard hanging off the back of my Kayak Boat. Not really, but you get the point. My mind naturally works that way sometimes, so I have to put a stop to it somewhere.



I haven't decided where to stop yet, but I think the top end would be the 6hp. The cost difference going to the 8hp is about the same, however, that's when you start to see the weight jump up from 57 pounds to the high 80's, I can't remember the exact number.



Current options are the 5 and 6hp, still got some more deciding to go. Part of me says, if there's no real difference, then save a few bucks and go with the 5. In reality, the difference is 1hp. I'm probably thinking about it way too much, it's not like I'm deciding between a 115hp and a 150hp, which would make a much larger difference.





Sorry for the long rambling reply, just my mind working overtime...at work.

I'd personally go with the 6 if the cost isn't an issue a 5 should get you planeing & a 6 will obviously be better. If your thinking you'll be stuck at displacement speed then you might as well go 2.5 as the extra hp wont gain you anything untill your planeing. The 8 wouldnt be my choice, a 2 cylinder 4 stroke isnt a particularly well balanced engine due to both pistons being at the top of the bores at the same time & they often employ balance shafts to counteract the inherent imbalance. The extra weight for 2hp isnt worth it imho, maybe if you were going to max out & go to 10hp then maybe worth it but in 4 stroke if the choice was 6hp single or 8hp twin I'd take the single for ease of handling. A different matter if we were talking 2 stroke as a 2 cyl 2 stroke is 1 up 1 down so they are more balanced but not the case with a 4 stroke

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[fastasfox]

10hp Mariner 2 stroke around 35 kilo in weight.
Powerful enough for you to have a bit of fun if your on your own and it will push you a buddy n kit along fast enough....but obviously don't expect to give Campbell a run for his money. They are reliable too....cheap to service cheap to fix cheap to buy.

[beamishken]

Originally Posted by fastasfox

10hp Mariner 2 stroke around 35 kilo in weight.
Powerful enough for you to have a bit of fun if your on your own and it will push you a buddy n kit along fast enough....but obviously don't expect to give Campbell a run for his money. They are reliable too....cheap to service cheap to fix cheap to buy.

If he's going for used 2 stroke he might as well go to the boats rated max of 15hp as its the same weight as a 10hp 2 stroke. I'd assumed he was looking at new engines & wanted 4 stroke though

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[CherMac]

9.9 hp in America to avoid limits on certain lakes rather than 10

[DH1]

Remember that if you go for bigger outboards, you will also need a separate fuel tank and trailing hose.
I suppose you need to consider your range as well really.

[DH1]

From what you are saying, I would go for the biggest single cylinder with its own tank. Just keep a small spare Jerry can up front.