Outboard engine size for a 500kg Rigid Raider (copy of Boston Whaler 17)

Iskander-M

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Hi guys,

Just bought another project boat, an empty hull coming from the Rigid Raider Mk1, no console or anything, the plate says “MOD 5.2M Dell Quay Marina Dory 10 men max or equivalent” (will post the pic later).

As far as I know (may be wrong) this is copycat of the Boston Whaler 17 hull that was adapted by Dell Quay for military production for MOD. Looks like this boat was built for the military use even before the RTK Marine bought the Dell Quay Marine with all the moulds. Wikipedia claims the RTK being the originator or the Rigid Raider so now it looks like this is wrong.

My question is about the min engine size I should go for. Looking at the RTK specs the max size was 140hp and 8 persons, boat weight 800kg (empty or with engine fitted?). I saw on other websites that the weight is 500kg, not sure.

I plan to use it as a tiller fishing boat maximising the deck space, no console or anything, taking may be 3-4 passengers max for fishing trips. What would you recommend guys?

Thanks in advance
 

Scott Danforth

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what does the data tag state is max hp?

if it doesnt, you need to calculate the max hp based on the OB formulas which account for lenght of boat, transom height and transom width.
 

Scott Danforth

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if it was rated at 140, then any motor from 75% to 100% of the 140 rating would be good

however if it wasnt rated at 140, then you need to go off the formula

The Rules are in 33 CFR Subpart D 183.51 - 183.53

Outboard horsepower ratings are based on a variety of factors. These include;

Centerline length;

Maximum Transom width.

Transom Height.

Type of steering: remote or tiller.

Hard Chine Flat Bottom or Other.

There is a formula for computing Safe Horsepower based on various combinations of the above factors. See the table below. This only applies to a monohull boat less than 20 feet in length (6.096 meters)

Canada uses similar formulas for computing outboard power in kilowatts. (1 HP = 0.745 KW)

The below formulas do not apply to: sailboats, canoes, kayaks, and inflatable boats, that are designed or intended to use one or more outboard motors for propulsion. It does not apply to boats that are true multihulls. A multihull makes two or more separate footprints in the water. A pontoon boat is a multihull.

Taken from Title 33 Code of Federal Regulations Subpart 183.53
Outboard Horsepower Capacity
Compute the Factor = Boat Length x transom width
If Factor is (to the nearest integer) 0-35
36-39
40-42
43-45
46-52
Horsepower is
3
5
7.5
10
15
Note: For flat bottom hard chine boats, with a factor of 52 or less reduce HP one capacity increment
If the Factor is over 52.5 and the boat has :

Remote Steering and at least 20" Transom Height
HP =
(2 x Factor) -90

No remote steering or less than 20" transom Height

For flat bottom hard chine boats
HP =
(0.5 x Factor) -15

For other boats
HP =
(0.8 x Factor) -25



Example 1: Bow Rider - Length = 19' 6" Transom width = 84" remote steering and 20" transom height.

(Length x Transom Width X 2)-90 = HP = ( 19.5 X 7.0 X 2) - 90 = 273 - 90 = 183 (136 KW)

Horsepower = 185 (138 KW)

Example 2: Length 12' Transom width = 46" Tiller steering and 15" transom height.

( Length x transom Width) = 45.96 Horsepower from table = 10 (7.5 KW)

Horsepower is rounded to the next multiple of five.
Transom width : is to be measured at the widest point of the transom including permanently attached parts of the boat such as rub rails.


Click on the image for the full size
Transom Height Boat Length
Boat length: Length does not necessarily include such things as swim platforms. However if the swim platform is molded into the hull and contributes to the buoyancy of the boat, it is included. If you need some more information on this call the Coast Guard at 202-372-1077.

Click on the image for the full size
Transom Height Boat Length
Flat Bottom Hard Chine means: The boat is flat bottom if you can lay a straight edge across it, and there is no vee or curvature. A hard chine has no curvature.
FlatBottom.jpg

Special Rules: For boats that meet the following qualification there is an optional test course method for determining horsepower:

13 feet or less in length.
Outboard Powered.
2 or less passengers.
Remote steering and 20 inch transom height.
 

Sea Rider

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If the boat is rated for a 140 HP motor and max 8 passengers capacity and plan tiller boating frequently with max 4 passengers don't need to go to max HP rated, 50-60% of it will suffice correctly propped for 4 up. Wich is the max HP and motor brand with a tiller available these days ?

Happy Boating
 

Chris1956

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Mar 25, 2004
Messages
27,033
I am not sure you can get a 140HP tiller model. I know the torque on my 150HP motor would make tiller steering unusable.

My neighbor had a BW 17 footer with a 90HP motor. It worked well.
 

Sea Rider

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Iskander,

Do you plan buying a second hand or a new tiller motor in the 50-70 HP range ?

Happy Boating
 

Iskander-M

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Messages
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Iskander,

Do you plan buying a second hand or a new tiller motor in the 50-70 HP range ?

Happy Boating

Mate, yesterday I bought a used Honda BF60 2010 with 400 hours on it and full history. The engine is controlled with side mount and I called Honda about a tiller. In the UK the tiller handle set up is available up to 30hp but you can separately order the tiller kit for up to 100hp.

When I looked at my BF60 Owner Manual all the pics are with tiller handle. The cost of a new handle for this engine will be about £400 plus VAT so we are talking £500.00 final price. I think it may work well with a fishing boat set up as the center console would jeopardise too much of deck space.
 

Sea Rider

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Right on mate,

Bit expensive but it's nice to count with more available space, a central console installation is a deck space killer. You can install a tiller extender and a high cushioned rear seat with locker and boat all day long....

Happy Boating
 

Iskander-M

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Messages
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Right on mate,

Bit expensive but it's nice to count with more available space, a central console installation is a deck space killer. You can install a tiller extender and a high cushioned rear seat with locker and boat all day long....

Happy Boating

Sure mate, this is the set up I want, just watched on YouTube there are versions of Verado 300hp with tiller so does not make sense to me to limit the engine size in the UK
 

Sea Rider

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A friend of mine has a 14.5 footer Boston Whaler with a light center console powered with a 2 strokes Tohatsu MC40 motor correctly propped for 2 up with a tach, achieves 6K wot revs which is the max rpm factory stated.

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This is a reinforced, resized tiller extender for my particular boating needs, excellent to boat all day long at speed while comfy seated. A tiller extender is usually used for trolling at low speeds would recommend to decrease the swivel bracket steering load to bare minimum for a smoother tiller driving.

Usually larger HP motors are for central console installations on larger boats, there are some few souls that likes tiller steering....

Happy Boating
 

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Chris1956

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Gee, a tiller 250HP motor. Who would have thunk it.

With the correct trim tab and other torque-cancelling accessories, I believe you could cancel the torque at a particular trim setting. However, every motor I tried had different torque at different trim settings. Most pull right when trimmed down and less or zero as you trim.

For Example, my current 135HPV6 Merc with a "Torque Tab" pulls right at planing speed, then zero and then left as I trim to max. Is their some auto-torque adjustment, that can be adjusted as needed?
 

Sea Rider

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Large tiller steer motors counts with an hydraulic steering system for a smooth and effortless tillering, If a standard portable size motor tires your hand badly on a long run, imagine those monsters....

Happy Boating
 

82rude

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First time I ever saw a big tiller boat I chased after him as best as I could till he slowed down and it was a 225hp merc! As Sea Rider stated it had hydraulic assist im sure.According to the vids ive seen the new merc system is absolutely effortless to use and zero fatigue.
 
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Faztbullet

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The combo is scary at full hammer down
Raise the motor and get rid of whale tail and it will perform better..the tail being buried kills handling
 

Sea Rider

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Sorry Fuz, nope on this one. That Tohatsu 40 has a larger set distance between both plates compared to very old motors. When this second hand combo was put into operation strange behaviours began to happen, The previous owner had the motor sitting way high on transom along an incorrect trim position.

Motor was manual trimmed to 90 deg and water tested on all type of local water conditions such as : calm, choppy, windy, the prop experienced excessive aeration except on calm flat water cond. You need to count with a combo that works spot on on most water cond, not only speeding at straight water courses which is not the type of boating I like doing.

So with my fiberglass techie chopped the transom 1.5 inches down, now water flow skims right under the upper plate. This is the ideal lower leg height (yellow line) for portablel and medium range Tohatsu motors and many other brands as well.

Leg-Transom Height.JPG

With respect to the doel fin installation, as long the fin rides paralell to water level with combo on plane and deck load perfectly distibuted will have 0 drag issues as the prop has been maximized to run to its full wot rpm range. I'm not a big fan installing doesl fins, any motor running strong, correctly height seated, well trimmed with a maximized prop will jump on plane at 3/4 throttle from a dead stop. Compared to before it's a day/night water performance difference way to go and boat forever happily.

Happy Boating
 

Iskander-M

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Thanks for your input guys, appreciate the conversation!

Just enquired about Honda BF50 and BF60 prices, this is basically the range I want to stay with.
The Honda BF60 is about £2000.00 more expensive for just 10hp up, so looking for valid reasons to bite the bullet but no one can reasonably explain the difference. Have to make my call...
 

Sea Rider

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The 50 HP is the older brother to the 40 HP, both are 800 CC motors, the 60 is a 1,000CC, a 200 CC difference along 2200 price difference.

If plan boating lightly with 2 up or medium loaded with 4 up, go for the 50 HP and prop it right for motor to run at least middle to full wot rpm range factory stated. Save the extra 2200 and invest it on a comfy rear seat, tiller extender with tach and a spot on prop maximization. Will still count with money left to drink many cold ones when in summer,


Happy Boating
 
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