The discussion below has some useful information on the inboard vs outboard debate…


mercan · March 11, 2011 at 11:09 am

Can you recommend an Impala (or any other boat) with an outboard engine for small passages like ~100NM? My impression with the outboards is that they tend to fail much more often when you need them. What is the cruising speed of the Impala with a ~9 HP outboard? Is it enough to overcome the tide in most cases?

KPALFREEMAN · March 18, 2011 at 6:35 pm

We bought our outboard two years ago. Our criteria was: min racing weight; plenty of power to cope with the tidal currents; sail drive propeller; charging if possible.

Initially I tried to find a used 2 stroke, since they are lighter than the 4 stroke for the same power, but no luck and/or the prices were high for the age of engines on offer. Most of them had high speed props, so would have had to buy a sailing prop.

So I checked all the new 4 strokes, at the cheaper end is Parsun, but heavy, at the top end is Honda, also heavy.

We settled on a Tohatsu 8.9 HP, short shaft (there are three lengths, take the shortest one), with sailing prop which is four bladed shallow pitch, and a 6 amp charging circuit.
My ramblings in “renewing an outboard” give the reasons for this choice.

It seems that the engine makers rationalise their production: for Tohatsu the 5 & 6 HP use the same engine block, 8 & 9.8 share a block, so they are the same weight and I guess the extra power is reached through changes to the air, fuel and ignition set up.

Perhaps the heavier engines are using their 15HP block for the 9.8/10 HP engines. Someone who knows lots about outboards can probably advise. Anyway the result is that the Tohatsu 8 HP & 9.8 HP are both rated at 37kgs.

The 6HP Tohatsu is rated at 26kgs so should be easier to manhandle, but we do not have any reports on how this engine performs in a headwind.

Engine power: with the 9.8 we seldom use full power, unless we are really late for the start ! Two thirds throttle give 5.5 knots, flat out about 6.5, which drops to 6 in a strong headwind.
We tow squibs and J24’s no problem, but drop to 4 knots when pulling a 1720.

I have also had an Impala with an 8HP two stroke Suzuki and that also worked fine, but top speed was 6 knots. Would pull two Flying fifteens, but struggled with three ! Drank petrol. The 9.8 four stroke seems to have only half the fuel consumption. Very roughly 1 and ½ litres per hour. I reckon on 50 miles on the basic 12 litre tank.
Much easier to refuel as no messing about with oil and of course the total amount of fuel to be carried is much less than a 2 stroke, probably more like a diesel.

We have raced and motored alongside an Impala which had a 15HP engine complete with electric start. They had about half a knot speed over us with our Suzuki 8HP.

Since the 9.8HP can get you up to hull speed, it is probably as fast as the 15HP.

Reliability: Tohatsu: Only problem has been the spark plugs, after a season’s use (est 80 – 100 hours) cleaning and re-setting them does not seem to work, just replace them. £5 each on ebay.
Even when the plug problem surfaced, the engine still started fine, just did not have any power.
This year, as well changing the plugs and oil, I plan to change the gearbox oil. Getting the correct oil, in correct tube, seems to be difficult, but local chandlery reckons its arriving “tomorrow”.
Of course if you have the motor professionally serviced that’s not your problem.

I think the golden rule is that the first time you have trouble that does not clear up with fresh plugs and clean petrol, get the motor professionally serviced. At least with an outboard you can shop around for cost and speed of service.

Anchoring via prop is of course easily resolved without getting wet !
The worst I had was a large poly mesh sack whilst towing three Flying Fifteens. Naturally they entertained themselves offering large amounts of gratuitous advice.

Never had any problems with the 8HP Suzuki – no oil to change, did not even change the plugs for four years ! Always started. Longest non-stop motoring trip was about 25 miles each year – but as noted you needed to be followed by a tanker.

A note on propellers: I once worked out the potential speed for the prop on our Tohatsu and at max revs it equalled 10 knots without allowing for slippage. Advice on slip varies widely from 25% to 50%, since the Impala is a very easily driven hull, then 2/3 rds power/25% slip = 5 knots, which seems about right.

Electrics: We have the standard 65 Ah battery, but its very old so probably only worth 30 or 40 Ah. Our normal pattern is once a week 35 mins motor, 2 hours sailing, 35 mins motor, with speed/depth/VHF/GPS and it runs a whole season no problem. Occasionally we use the LED nav light, and sometimes sail all day. In the first season I took the battery off mid season to charge it. But after about half an hour the charger dropped to trickle, so did not bother last season and the battery is still just on 12 volts. (VHF and GPS both claim to work from 10 volts up)

There is an article in the Jan 2011 PBO about making a circuit to fit to Tohatsu’s so that they will charge your battery, but I guess this refers to older models as we just connected the leads from the engine to the battery before we heard there might be problem and everything seems to work fine. The circuit diagram in the instruction book refers to “REC/REG” which it shown attached to the battery and I suppose is a rectifier and regulator for charging the battery.

Justin Philipps · February 8, 2019 at 10:10 am

We have a Tohatsu 6Hp 4 Stroke which pushes Scallywag along quite happily. We get 6knots max but only use less than half the fuel with our old unreliable Mariner 2 Stroke 8hp.
100% reliable the only issue we had was a grungy carb when I didn’t winterise properly and drain the carb.

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.