What size outboard for an ILur dinghy?

[dave2700]

Hello I'm planing to build the ilur dinghy but I want to know the cost of the outboard before hand and for that i need to know how many hp?

I will be using the Ilur in west scotland for fishing trips and such. There are strong tides here so what size outboard would you recomend?



Yes i will also have the sailing rig but i want to be able to have the outboard drive the boat eficiantly for a couple of houres without needing sail so I can take off the mast when I'm going fishing.


I know he says 4hp but I just want to be sure.

https://www.vivierboats.com/en/produ...nker-kit-ilur/

[wizbang 13]

you'd trust us more than the designer ?

[dave2700]

you'd trust us more than the designer ?

No. But I want to hear what you people think. Becouse that might be as an emergency engine for when under sail not as main propulsion.

[wizbang 13]

4 hp is plenty. I recon 2 hp is plenty.That would still give you at least 4hp per ton.
Fighting current is math, not power.
Fighting wind and waves...that takes extra power.

[Ian McColgin]

The HP is limited by the shape and volume of the stern. The extra weight back there will put the stern down a bit. Should be less than 60# but still. And as you get up in the 5kt range the stern will squat. Finally, you'll be mounting it well off-center to clear the rudder, more like so the rudder clears the motor. You want it light enough for safe shipping and unshipping.

"High thrust" outboards are your way to go. Different brands use slightly different mixes but in essence the gearing is such that for a given engine rpm the prop spins slower than the same outboard that's not high thrust. The prop is pitched a bit steeper and often has a greater diameter. High thrust is hard to find in motors less than 5HP

While not "high thrust", Honda makes a great little 2.3HP that people around here use as a trolling motor to save the main engine. Great unit. It has no neutral and reverse is achieved by turning it 180 degrees. It takes a little getting used to but is lovely and safe.

Very few under 5HP outboards have any possibility of remote steering. Simply clamp a bit of stiff hose over the handle to get a bit further forward. And extend the kill tether.

G'luck

[David G]

No. But I want to hear what you people think. Becouse that might be as an emergency engine for when under sail not as main propulsion.

What I think is your should listen to the designer.

If it was a vintage design with no clue as to powering it... I'd agree with Wiz. 4 hp max. 2 hp minimum.

Oh, I see Vivier calls for 4 hp. Makes sense.

[Autonomous]

I've been asking around lately and though I have no experience with anything smaller than 6hp have formed an opinion on the subject. That little Honda 2.3 is very light weight and from all accounts, very loud. All the little ones are loud, some are just louder. The designer of the CLC Southwester Dory says 2.5 hp is enough but a 4hp motor at half throttle is much easier to live with.
The Tohatsu-rebranded Mercury 4hp engine has through hub exhaust which would help with the noise level and moving exhaust gasses away. That would be my 1st choice.

[Ian McColgin]

A dear friend after cruising with me for a couple of seasons got as her first boat a Cape Dory Typhoon. The little 2.5HP that came with it pushed her one ton to 5 kts, just under hull speed. Small and simple is what you want. The designer's suggestion is a maximum, not minimum, and for this boat the performance difference between two point something and four is minimal.

[Clarkey]

Given your aim for fishing trips lasting a couple of hours I would go for the smallest motor that offers the option of a remote tank connection - usually 4hp and up. The smaller engines will require you to lean over the back of the boat to refuel their integral tanks every hour or so. Not something I would fancy doing if the boat was moving around.

[Autonomous]

X2 on the remote tank.

[dave2700]

The HP is limited by the shape and volume of the stern. The extra weight back there will put the stern down a bit. Should be less than 60# but still. And as you get up in the 5kt range the stern will squat. Finally, you'll be mounting it well off-center to clear the rudder, more like so the rudder clears the motor. You want it light enough for safe shipping and unshipping.

"High thrust" outboards are your way to go. Different brands use slightly different mixes but in essence the gearing is such that for a given engine rpm the prop spins slower than the same outboard that's not high thrust. The prop is pitched a bit steeper and often has a greater diameter. High thrust is hard to find in motors less than 5HP

While not "high thrust", Honda makes a great little 2.3HP that people around here use as a trolling motor to save the main engine. Great unit. It has no neutral and reverse is achieved by turning it 180 degrees. It takes a little getting used to but is lovely and safe.

Very few under 5HP outboards have any possibility of remote steering. Simply clamp a bit of stiff hose over the handle to get a bit further forward. And extend the kill tether.

G'luck

thanks for the help I will look into hight trust motors

thanks to everyone else too just can't reply to everyone

[Ian McColgin]

My impression is that 'high thrust' engines are more suitable for large displacement hulls at subdisplacement speeds. Either there is no market for or no economic need for low (under 3HP) motors for lightish craft.

[Edward Pearson]

My Ilur plans recommend a 2-2.5hp motor and a long shaft at that.

Ilur has more freeboard than almost all other dinghies which is why she has so much space and interior volume. Two more planks on Aber from where she derived.

He reckons you'll need to weight the stern down to make a short shaft work without cavitating which will be less than ideal. I think the Barnes has a removable cut out section to use a short shaft for this reason.

So you want a long shaft or medium. Anyway I'd carefully measure it off plan and know what you're buying.

There are a few Honda 2 longshafts about but not many. You could just buy the longer leg if you wanted it but I don't think they sell it now. The old Yamaha Malta 3.3 longshaft was pretty popular for this reason and its light but a stinky 2 stroke. The Yamaha 2.5 is quiet as it's not air cooled like the Honda. You can usually get a longshaft from a sailor selling up that used it as an aux and tender engine for a 20-30fter. Keep an eye in you local Gumtree classifieds etc. That's where they turn up. Expect to need to carb clean it and rejet it as the fuel clogs em up and a new plug/ gear oil. Stuff's been idle for a few years now with Covid.

Really electric's where it's at but you'd have to figure if you'll get the range you need. Its much more pleasant without the noise. You'll catch more fish under sail and oar though, less noise to frighten them off. Just chuck a lure over as you amble along! 42cm minimum sea bass size and max 2. You should catch and realease 'em all really as there under pressure.

[Andrew2]

Somebody was selling a long shaft kit for a Honda 2.3 over on the PBO forum, quite reasonable. So easier than trying to find the LS motor.
OP says it is auxilary. i.e. getting home if the wind quits. So the need for an external tank is not so important. Sub 4hp don't usually have the option.

Oops, just notice the 2hr comment.

[Autonomous]

I will be using the Ilur in west scotland for fishing trips and such. There are strong tides here so what size outboard would you recomend?

Yes i will also have the sailing rig but i want to be able to have the outboard drive the boat eficiantly for a couple of houres without needing sail so I can take off the mast when I'm going fishing.


I know he says 4hp but I just want to be sure.

https://www.vivierboats.com/en/produ...nker-kit-ilur/

Find some YouTube videos of the little Honda 2.3s in action. That horrible racket for hours at a time, even with earplugs which would affect your ability to hear other important noises, would be awful. It's forte is a lightweight, easy to handle kicker to get your dinghy out to your moored boat. Not a multiple hour fishing trip fighting strong tidal currents. Ugh.
They tell me the Suzuki 2.5 is much easier to live with, though still rather noisy.
Water in the gas... or worse, would be a patiently waiting specter when the outboard's 1 liter fuel tank needed topped up under way. Not something you want to do in sporty conditions.
Listen to Vivier, he knows the boat.
Consider Tohatsu's 4 hp with through hub exhaust and optional external fuel tank as you will be working the boat in serious seas. Were you a daysailor I'd suggest the Suzuki 2.5.

[dave2700]

thanks i wont be buying an engine before I am not atleast halfway trought the build. And thanks for the tip with a long shaft or medium.

[Edward Pearson]

Manual for the Honda is here...

https://www.honda.co.jp/ownersmanual...ope&model=BF2A


Short shaft - Transom height 38 cm above LWL

Long Shaft - Transom height 51 cm


(Anti-cavitation plate should be 10cm below surface)

Ilur is designed for 3 people's mass, so solo she'll be sitting a bit higher than the LWL shown on the plans, though the engine weight and your weight will submerge the aft sections...the transom is just above the LWL with sinusoidal aft waterlines so you still might avoid transom drag, but you'd want a tiller extension and should try to sit on the midships thwart.

Don't use my Honda anymore but I'd use John Welsford's method of squeezy refuelling bottles to put more tonic in if I had a long run planned. Never be without some oars. The Ilur ones are pretty special having two gear positions for normal or into the wind. That and the Yuloh socket and sub floor storage for it all. I'd still go to the trouble of all that. Oars are still much easier to carry than a Honda 2hp.

[Giznuk]

Having read 2/3 of the replies.
I have an ILUR and a 2.5 Suzuki, but also a Century Seagull (4 hp).
The 2.5 long shaft Suzuki is plenty under light winds, why else would you use her? Well needing to get out an inlet with an opposing tide... has yet to be done by me, especially with incoming swells. Hope to add to this thread as my experiences build.
I have added a bracket just 2" below the transome height but haven't tested either on it yet.
The British seagull is direct drive and, seems to be too much. More as I work more with her. But I can imagine trying to get out that inlet at idle and possibly having too much with those 5 blades pushing over the sharp chop? And along those lines I'm wondering what the lack of thrust might do in the same conditions with the 2.5, will the blades stall with the pushback?
I beleive the 2.5 will be plenty and is a nice "clean" engine! photos of bracket....
Also, the bracket in the below photo is off a Catalina Capri 165. It provides clearance so you can lock down the motor and use your rudder (full down possition) without fear of a prop strike. The tiller then allows you move forward and make your hull more ballanced/efficient so the one quart of gas goes further up the river to you overnight spot! As what happened here:
https://www.youtube.com/watch?v=7ieKOP_Ohz8 youtube, green is me - per bracket and you can clearly see the need to be a bit lower than the transome. Though the transome does work as when you are aboard, it is lower. But if you move forward to grab something, the prop cavitates

[P.I. Stazzer-Newt]

Scotland's west coast has tides, sometimes serious tides...