Bob, Keep in mind the prop size has to be about the size the maker reccomended to keep the upper engine speed at the makers reccomended upper RPM. The company normally acts as broker for the vendor, who unless otherwise stated is not selling in the course of a business.
So what is the smallest (shortest length) boat with an inboard you have seen or read about?
The 'push boats' for the Maine Windjammer schooners are about 14 feet long with honking great engines, and LIBERTY, a Hodgdon built commuter yacht has an inboard dinghy maybe 12' long. You may need to define the question further to get what you are looking for -- are you asking in an historical sense, or regarding modern motors and designs? From the various discussions on the forum, it seems that motors have gotten larger and produce higher rpm's than 100 years ago, thus limiting their use as inboards in small boats.
The introduction to our host's Fifty Wooden Boats has the lines, construction and offsets for the Irreducible.
There has been some interesting discussions on how to use belts to reduce the rpm on inboard engines on the Rescue Minor threads here -- well worth checking out! So there is an easy way to get low rpms, high torque, and steady running speeds out of a short stroke 2 stroke engine without re-maching crankshafts etc? Strikes me that far more satisfactory power for Irreduible would be an electric trolling motor embedded in the rudder. One could certainly use a trolling motor in the rudder (or instead of the rudder for maximum simplicity), or alternately a DC motor in place of the inboard gas engine. That would be the designer at the helm, I'm not really unhappy just trying not to scratch the varnish. Contact us with errors, copyright information, problems or suggestions to our plans or DIY info. Based on the Donzi Sweet 16, this little treasure boasts a generous deep V at the transom allowing for smoother running and bad weather handling. We are happy to tailor this vessel for your inboard, or modify the transom to suit your preferred outboard. Please visit our Deep V 16 page to see build images and links to videos of this boat at speed. The plans and kits posted here are the lawful property of the designers or heirs who posted them.
Get Ready to FlyWhen you hear the roar of automotive engines and catch a glimpse of giant roostertails, chances are pretty good that inboards are on the water. 1 Litre ("Y" class) hydroplanes are small, light and lightning quick, racing to speeds over 105 mph.

2.5 Litre Stock hydroplanes ("S" class) race to 100-plus mph, using 145 horsepower marine-based 2000 and 2300 Ford engines. 5 Litre hydroplanes ("E" boats) race to speeds of 115 mph, powered by mildly modified marine-based stock 305 Chevrolet and 302 Ford V-8 engines (285 horsepower). Crackerbox ("P") is one of the oldest Inboard Runabout classes, distingushed by the 2-person team of driver and rider seated side by side at the rear of the hull. Grand National Hydroplane, aka GNH or 7 Litre, is the largest non-supercharged Inboard hydroplane class. K Racing Runabouts (K-Boats) are a professional class of propeller-driven Inboard Racing Runabouts, using virtually unlimited engines and fuel. Pro Stock Runabouts (PS) are a professional class of propeller-driven Inboard Racing Runabouts. In each of the sections below the pages have NEXT and PREVIOUS buttons at the bottom of the page.
A few years ago some rot was discovered in the port garboard plank in the aft third of the boat.  Because of the Skiffcraft construction method with the keel placed on top of the two garboard planks (compared to a Lyman where the garboards are fitted to the keel), both garboard planks were replaced at the same time (a total length of approximately 8 feet starting at the transom) and the removed section of the keel.
This past spring we were commissioned to perform some minor structural repairs and refinish the boat in preparation for one last season of use and listing for sale of the market.  You can see photos and discussion of that project here on our website. The 275HP Chrysler 360 runs very well and was professionally examined and seatrialed prior to delivery to owners this spring.  The boat also includes a tandem trailer with 4 new tires replaced this spring. Come visit LakeShore Electric Boating at Moes Marine Service to explore the Vermilion Harbor with family and friends! These props tended to be square that is had a pitch equal to the diameter or even a little more pitch. Whilst every care has been taken in the preparation of these particulars, the correctness is not guaranteed, and they are intended as a guide only and do not constitute a part of any contract. To get WoodenBoat delivered to your door or computer, mobile device of choice, etc, click WB Subscriptions. The latter would require a certain amount of thinking to work out the batteries and speed control, but it would work fine and be almost silent. Available as an outboard and inboard it an easily tow-able boat that will bring you home and take you out with a minimum off fuss. WoodenBoat magazine nor any of our staff or agents bear any liability concerning the correct ownership or inherent safety of these designs or boats. As the largest category in the APBA, Inboard racing classes include both runabouts and hydroplanes that are fast, fun and LOUD. These small, light and racy hydros are a great starting point for young drivers or for budget teams.

Engine modifications are limited, putting a premium on driver skill, with great competition at every event.
In competition, the boat must carry a crew of two (driver and riding mechanic) seated side by side, strapped inside a steel tubing roll cage.
Many of the inboard designs have outboard options and there are designs in the Cruiser and Work Boat sections for inboard power. These engines had lever controlled timers, the little 1 Hp with lever behind the flywheel and all others a short handled lever at rear of engine on the output shaft.
Electric Propulsion For Boats by Charles Mathys, or Electric Boats by Doug Little (now out of print but available used) cover the subject.
Classes span from wild Jersey Speed Skiffs (70mph) to supercharged 1,500 horsepower Grand Prix Hydroplanes (170 MPH). Nobody plans to flip, but it does happen from time to time…and when it does happen, the training pays off.Inboard Runabouts, or flatbottoms, come in both the open cockpit and capsule variety. The 300-plus horsepower Skiffs are a wild ride; they spend a lot of time airborne, bouncing around, rolling sideways to seemingly impossible angles, at 80 MPH. Be sure to check out these sections for if you are interested in designs for inboard power. OTOH, the experience of electric power is very different from running a small slow-turning gas engine - better or worse is your call.
Flatbottoms are known for their long lean shape, which allows the boat to slice through water as they move at speeds of 100+ MPH. Some runabouts run with two racers on board – a driver and a mechanicHydroplanes glide over the water on a cushion of air, with only the propeller and sponsons on each side touching the water while air rushes under the hull, giving the boat the “lift” it needs to skim across the water.
The Strelinger was a very different engine and was not affected in design by this buy out, but you are quite right about the engine being a Detroit-Strelinger !! These bad boys are fast, with some classes running at speeds in excess of 150mph - Inboard hydroplane classes range from 1 liters (T-Boats) to the 1500hp supercharged Big Block Chevrolet V8 Grand Prix (GP). Controls are similar to those of an automobile – steering wheel, accelerator pedal, rear view mirrors – but no brakes!WATCH OUT for roostertails!
If an inboard hydro is in front of you, chances are you’re about to get blasted with a wall of water from the giant roostertail of the boat in front of you.
Racing into a turn favors the leader, with those behind having to deal with roostertail fallout!

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