jon070

I recently started building a 1/10 shovel nose, however, I am not sure on which power plant to use. I will be lightening the wood frames and using 1/16 for skin on the bottom and 1/32 for skinning the top. I am estimating the hull will be just a touch under 3 pounds with no hard ware or radio equipment. The hull is 35 X 15 inches

Currently I have been running a 1/12 shovel with the fifty dollar Aqua engine with all vegas hardware it has worked out pretty well, just a bit of a tail dragger. I have been racing this with the dueces and has done pretty well.

I don't care for the dynomite engine so any other suggestion would be welcome.

Thanks

Jon

RickeyRockstar

My Feedback: (4)

OS or K&B marine are the best prob a 7.5 min K&B 11cc inboard or a os 46 Nitro marine

Hydro Junkie

On your boat, it falls size wise into the .21 catagory in NAMBA, APBA and IMPBA. Not sure where RickyRockStar got his numbers from, but OS and K&B aren't the best choises on the market. Both the OS 46 and K&B 45 require a 180 degree header so in your boat, they would be hard to set up unless you run the tuned pipe under the deck. The K&B 67 is hard to get, is difficult to get spare parts for and is overkill for any boat under 40" long, so I wouldnt' look at that one either. You would be best served with a .21 with the exhaust over the flywheel. If you want to move up to a 36 to 40 inch boat, then a .45 would be a better fit. You might want to send a PM to Ron Olsen since he lives in Michigan as well and knows what's going on more so than I do.

R.J. West

I believe a .21 wouldnt do a 35 inch hull justice.....and a .60-67 would be OVERKILL![img]file:///C:/Users/Ron/Pictures/rock2.gif[/img]

Hi jon070!
Is it possible if you can upload a pic of your hull? Mainly the interior so we can get a better visual. Is your boat built from a kit or is it a scratch build?

I have build a Dumas "Miss Unlimited" which is also 35 X 15 all wood shovelnose, it calls for a .40-45 engine.
Your hull specs lay within sport 40 hydro regulations...If theres enough room, and I believe there is, I would go for a O.S..46 marine.....A less costly option is to check out www.mecoa.com.
They have a good selection of engines .40 thru .45......Check out the Mecoa .46 marine....Not bad specs for a $99.00 engine.....

Hydro Junkie

RJ, you're forgetting something. Your boat is constructed with heavier wood than what is going into Jon070's round nosed boat. Dumas also did that instruction sheet when engines had considerably less power than they do today. These both must be kept in mind when recommending an engine. I've included some info that may help in choosing an engine.
This is copied directly from the NAMBA rule book:

C. CLASS SPECIFICATIONS
1. Sport 21 Hydroplane
a. The class will be named Sport 21.
b. Minimum hull length will be 27�.
c. The engine must conform to Class A specifications, see Section 10 – rule A.1.

2. Sport 40 Hydroplane
a. Minimum hull length will be 35�.
b. Engine Specifications
i) Division 1
(a) 5.74 to 6.56 cc (.35 to .40 cubic inch), front intake, side
exhaust, no full wave tuned pipes. Speed control must be by
rotating barrel carburetors. Carburetor bore diameter is limited
to 7.67 mm (.302�) for all engines except for non-schneurle
ported engines, which may use rotating barrel carburetors of up
to 7.92 mm (.312�) bore diameter. All carburetors must have
needle valve and/or fuel discharge nozzle extending into the
carburetor bore for a minimum of 3.04mm (.120").
(b) The carburetor will be physically and dimensionally checked.Winning boats may be checked at the completion of day’s
racing. If infractions are found, the boat will be disqualified
and the next boat moved up.
(c) A visual inspection by a race official for compliance to these
rules will be made any time during the event. Boats not
complying with rules will not be allowed to run until corrected.
ii) Division 2
(a) The engine must conform to NAMBA Class B Specifications,
see Section 10 – rule A.1. Tuned pipes are allowed.

A. ENGINE CLASSIFICATION
1. Internal combustion glow engines:
Class Displacement
A 0 - 3.509 cc (0 - 0.21 cu. in.)
B 3.51 - 7.509 cc (0.22 - 0.45 cu. in.)
C 7.51 - 11.009 cc (0.46 - 0.67 cu. in.)
X 11.01 – 30.009 cc (0.68 - 1.80 cu. in.)

Hydro Junkie

From the IMPBA Rule Book:

Sportsman 40 Hydro (Sport 40 Hydro)
Boat Specifications
1. Boat must be inboard powered.
2. Hull must be a three (3)-point hydroplane configuration and resemble a limited or
unlimited hydroplane design of past or present, except outrigger, modified outrigger,
tunnel, or canard hulls are NOT permitted.
3. Boat must have a name, sponsor's name, logo, and/or a racing number affixed to hull
(a local, national, or fictitious sponsor name is acceptable).
4. Hull shall be a minimum of 35 inches and a maximum 40 inches in length.
5. If the bow is recessed behind the tips of the sponsons, that recess shall be no larger
than 25% of the overall length of the boat.
6. Boat must have a driver and/or simulated enclosed cockpit.
7. No part of the strut may protrude behind the transom of the boat, or have a width
greater than 9/16 inches or a length longer than 3 inches.
8. See Sport Hull pictorial for clarification.
Engine Specifications
1. IMPBA D class engine shall be used.
2. Twin engine installations are not allowed in Sport 40.

Sportsman 20 Hydro (Sport 20 Hydro)
Boat Specifications
1. Boat must be inboard powered.
2. Hull must be a three (3) point hydroplane configuration and resemble a limited or
unlimited hydroplane design of past or present. Hulls not permitted: outrigger,
modified outrigger, tunnels, or canard hulls.
3. Boat must have a name, or sponsor's name, or logo, or a racing number affixed to the
hull. (a local or national or fictitious sponsor name is acceptable).
4. Hull shall have a minimum length of 27 inches, and a maximum length of 35 inches.
5. If the bow is recessed behind the tips of the sponsons, that recess shall be no larger
than 25% of the overall length of the boat. (refer to Sport Hull Configurations)
6. Boat must have a driver in open cockpit or simulated enclosed cockpit.
7. A strut width no greater than 9/16 inch wide may be mounted either under the hull
or on the transom with the farthest end of the drive dog, as measured from the
transom not extending out more than 3 inches.
8. See Sport Hull pictorial for clarification of Sport Hull Configurations.
Engine Specifications
1. IMPBA class B engine allowed in Sportsman 20 Hydro.
2. Twin engine installations are not allowed in Sportsman 20 Hydro.

Engine/Motor Definitions and Classifications
1. Displacement
The cubic inch displacement of a power plant shall be the sum total of the
displacement of all cylinders in the power plant. The cubic inch displacement of multicylinder
engines, two or more engines driving attached together and two or more
engines driving separate propellers will be added together to determine the total
displacement and thus the class the power will run in. To determine engine class, read
calculated engine displacement to three decimal places, drop all remaining figures.
2. Internal Combustion Engine Classes
Class Cubic Inch Displacement Metric Class
A 0.000 thru 0.129 2.1 CC
B 0.130 thru 0.219 3.5 CC
C 0.220 thru 0.300 4.9 CC
D 0.301 thru 0.458 7.5 CC
E 0.459 thru 0.670 11.0 CC
F 0.671 thru 1.830 30.0 CC
Formulas: CC X .061 = CID, CID divided by .061 = CC

Hydro Junkie

Per the above rulebooks:

The OS .46 is ILLEGAL IN NAMBA BUT LEGAL IN IMPBA SPORT 40 HULLS

For K&B engines:
the .21(model 8916) is legal for NAMBA Sport 21 and IMPBA Sport 20 hulls.

the .40(model 4012) is legal for NAMBA Sport 40 Division 1
the .45s(models 7525 and 9181) are legal for NAMBA Sport 40 Division 2 and ALL IMPBA Sport 40 hulls

K&B SPECIFICATIONS
Model 8916
Size 3.5cc
(.21 cu. in.)
Bore 0.65
Stroke 0.64
H.P. 1.25
RPM 2000~25,000
Weight 9.9 oz.
MECOA Price $192.00 ( listed as available 1/1/2009)

Model 4012
Size .40 cu. in.
Bore 0.84
Stroke 0.72
H.P. 1.0
RPM 2000~13,000
Weight 16.5 oz.
MECOA Price $165.00 (listed as in process)

Model 7525
Size 7.5cc
(.45 cu. in.)
Bore 0.85
Stroke 0.8
H.P. 2.75
RPM 2000~25,000
Weight 34 oz.
MECOA Price $276.00 (listed as in process)

Model 9181
Size 7.5cc
(.45 cu.in.)
Bore 0.87
Stroke 0.76
H.P. 2.2
RPM 2000~25,000
Weight 14.7 oz.
MECOA Price OUT OF STOCK

OS .46 SPECIFICATIONS
Displacement: 7.45cc (0.455 cu in)
Bore: 22.0mm (.866")
Stroke: 19.6mm (.772")
Power Output: 2.5 BHP at 23,000 RPM
Weight: 445g (15.7oz)
Practical RPM Range: 2,500 - 28,000 RPM
Crankshaft Thread Size: 1/4-28
Length: 106.0mm (4.17") from carb intake to the front of crankshaft
Width: 36mm (1.42") neglecting the engine mounting flanges
45mm (1.77") between the mounting hole centers
Height: 87mm (3.43")
Cost at Tower $249.99 plus shipping

jon070

Thank you hydroJunkie I searched and searched for that information for the past week. The hull im building was origially for an electric http://www.astecmodels.co.uk/plans.htm. I have to admit I really like that aqua 18 and I have collected a few of them over the summer so my origanl plan was to use one of them. However after getting my frames cut and assembled I realized I was going to be in trouble. I was leaning more toward the 21 anyway. The Rossi 7.5 seemed to be a little more than I needed.


Could I use a 3/16 flex shaft on the 21 motor?

I will throw some pics up this weekend.

THanks

Jon

jetpack

My Feedback: (1)

Look around you might be able to find an old OPS .21 side exhaust brand new somewhere. Very good motor for it's time and can be found at a somewhat decent price. Not sure if the header would stay inside the cowl not.

If you build light it might be something to think about. The roundnose remember won't be as fast as hull as the newer ones also so maybe that will help that type of engine hull combination.

I'm curious myself. I have a side exhaust with manifold plus a MAC's header if you need measurements and help let you know where to look for one.

The 3/16" flex shaft is the most common in the .21's, most go without the teflon tube and just use the brass tube.

jon070

Okay here are pics of my summer creation and the start of my winter creation. Also putting together some 12 riggers with my aqua 18 arsenal.

sanecrazy1

My Feedback: (1)

Look what a CMB 90 does for a 38" Madam-X LOL http://www.namba.com/content/race_re...D=73&ClassID=8


Yeah the lil yellow fellow kicked some tail on the twin LOL


Who says bigger is always better

JWMods

I might have the perfect engine for it and its 5cc. Yes, its an O.S.30VG-P(X) that I modified for use on a Thunder Tiger
O/B geared lower unit. The engine was ran in our clubs "Open Tunnel" class and kicked butt on several modified 7.5cc O/B's.

The major hurdle would be working out a water cooled head for it cause I ran it as aircooled and it would need a
non-pullstart backplate

The engine was run on a Lynx tunnel and was close to more than the Lynx could handle. It was limited to smooth water
operation and it was hard on the Thunder Tiger O/B flexshafts,, so much so that the engine had to be shelved.

I used the stock aircooled head with most of the fins turned down (on a lathe) and had to turn over .055" off the button
seat where sits on the sleeve flange to get .008" headspace with .020" sleeve shims. The crank induction window was also
opened up to 215* total open duration. It's without a doubt the most user friendly engine I have ever ran, and a powerhouse
at that.

I don't have a good picture other than photo's but there is a pic like it in one of Toyotatruckin's (I think it is) topics back about
5 pages in the nitro forum. It's a pic from Topfuel4430 (not exactly sure there) of his O/b that he made up,, maybe Toyo can
help here.