3.5 HP 4 Stroke outboard

My Outboard Survey indicated that there are a number of owners using 3.5 hp engines, particularly in Falmouth where they remove the outboard when racing, but I do not have any detail on how well they work.  As you have found, most have only fwd & neutral gears and reverse is achieved by rotating the engine.   I have a 2 stroke Mariner 5 that has full 360 deg rotation, but the boat geometry prohibits little more than a few degrees each way.  The main problem is the proximity of the stbd cockpit side that fouls the tiller, but the bracket is also low in the well and the bottom of the hood fouls the locket lids.  Although a 3.5 powerhead is smaller than a 4/5, it does not look that much shorter, so I would be surprised if you are able to rotate the engine more than about 20 deg each way. 

Perhaps someone using this size of engine can confirm whether I'm correct? 

Regards

Keith

i have a 5hp honda.  while coming up the river against the last of the rising tide and a 20 mph headwind i came to a stand still several times.  it was a very slow go.  i usually would have just waited , i thought i could handle the tide but when the wind shifted and the river turned i just stopped,  with full throttle i barley made any headway.  finally the tide came after me and pushed me home.   other times it's been all the motor could do to left me off the dock when the tides peaking.  the current runs 4 to 6 knots i'm told.

other then these extremes the 5hp has worked well.  and it fits.

wm

Hi Cap'n,

I have a 4 hp four stroke Mercury.  At full throttle it moves the boat at 5.4 knots, at 3/4 throttle 5 knots.  I once had to borrow a 6 hp 2 stroke Evinrude when my Merc was on the fritz that probably got the boat up to about 6 knots.  The fastest I have ever gone under sail  (broad reach, full sail, gust of about 15 to 20 kt, two aboard) was 5.9 knots as measured in water with no current running by GPS.   The hull speed based on wl 17ft 3in is calculated at 5.4 kt by the customary formula ((1.3)*sqrt(WL)=hull speed in kts).  Thus my 6 hp outboard was getting my boat into a semi plane but this process has practical limits (squatting of the stern would limit this). A corollary to this is that 5-6 knots is also the highest speed of current you could overcome without having the boat planing!

I think 4 to 5 horses is probably sufficient; at 3.5 you are probably slightly underpowered (but should still be able to move about in flat water at up to 4.5 to 5 kt or so).  The main problem is not really driving your own boat against a current; where you need additional power beyond what it takes to just move the hull slightly beyond nominal hull speed in flat water and no wind is a) if you have something in tow, like a dinghy or another boat, or b) if you are trying to maintain your speed, or indeed your static position, against a big wind and/or seaway under bare poles or with some sail up;  these situations have in common that the resistance to forward movement by the boat is not only due to the hull friction and wake production, but also to some additional drag from another source.  That's why the supplied inboard is rated at 9.9 hp, for the safety factor in a storm or towing.  People don't use outboards that big because they won't fit in the well and they are too heavy to be convenient.

The 3.5 hp merc weighs less than the 4 hp merc, and the 4 and 6 weigh the same and are almost the same motor; the 6 does not give 6 honest horses, I understand that it is just tweaked to give a higher peak RPM, but the RPM on the water are actually determined largely by your prop anyway.  The big disadvantage of the 3.5 merc is that it has no reverse.  I seem to remember that some other manufacturer's 3.5 hp four stroke does however have a reverse gear...this would be highly to be prized.

Incidentally, I also have a 12 volt minnkota trolling motor with 55 lb of thrust (equivalent to about 1/2 hp) that moves the boat at a blazing 2.7 knots in quiet water! I have used it when all I wanted was something to get me out of my slip and beyond my club breakwater and I wasn't doing any cruising, and for this it is quite practicable.  Wouldn't do this in any real blue water situation however.

Best
Bruce (#199, Starfish)