Now that we have the basic anatomy of the prop covered, let’s look at how we can go about insuring that the optimal prop for your boat is installed.

Getting the right prop on your boat is very much a process of trial an error. The best place to start is to look at what you have on your boat now.

When your boat is at wide open or wide open throttle (WOT) what is the maximum RPM? Does this RPM fall between the manufacturer’s recommended range? If it does then you most likely have the best prop for your boat – if not, there’s work to be done. The goal when selecting the optimal prop is to have your maximum RPM for the engine fall between the range given by the manufacturer. The higher the RPMs you have within the range, the higher speeds you will most likely reach. Having the correct prop will deliver maximum speed and optimal efficiencies. However, if the RPM exceeds or falls short of the range, it will cause unnecessary wear and may damage the engine so it’s very important that your boat is propped correctly.

Once you have figured out what you’re getting for RPM with your current prop, it’s time to determine what size prop you have. Typically the prop size is marked on the side of the propeller, but you may need to remove the prop as manufactures sometimes put the markings on the hub or the closest part to the engine.

While you have the prop off I’d also recommend noting the number of splines there are on the propeller shaft as these can vary from manufacturer and between engine type (2-stroke vs. 4-stroke). You’ll also want to measure the gear case size. This is the outside dimension of the casing where the propeller abuts.

So, now that you know your current RPMs and prop size, what next? If your current RPMs fall within the engine manufacturer’s recommended range, than I would recommend nothing – stay with what you have. But, if it’s below, above, or you’re a “tweaker” then just focus on the pitch as the diameter is generally pre-determined by the engine manufacturer.

Propellers are sized in two inch increments; 17”, 19”, 21”. Generally a 2” pitch change will result in an average change of 300-400 RPM at full throttle. When thinking about pitch – think of it in terms of gears on a car. Selecting a higher pitch propeller will decrease RPM at full throttle, just like when shifting up to a higher gear in your car. Transversely, going to a lower pitch propeller will increase the RPMs.

Higher the Pitch = Lower the RPM
Lower the Pitch = Higher the RPM

Posted by ribcraftusa at 12:57 PM | Permalink | Comments (12)

I’m sure we can all agree that a Prop is a fairly integral part of the boating experience, but if you’re like me you don’t give them a whole lot of thought – they spin and make the boat move – good enough. All joking aside – there’s actually quite a bit to them and they’re more important than you might think. The prop is a critical element and having the properly (no pun intended) sized prop on your boat is key to performance and engine longevity. Folks have asked some great questions recently regarding how to select the right prop for their RIB, so I thought we’d look into it together.

Before we can even talk about selecting the right prop, I think it would be helpful to explore the anatomy of the prop, something that I like to call “Props 101”. So here are the basics.

Like people, propellers are either left or right handed. Left handed props rotate counter clockwise and right handed props rotate clockwise if looking aft. In addition, a prop has a leading edge and a trailing edge with the leading edge obviously being the part that first cuts through the water when spinning. The trailing edge is where the water flows off the prop.

Propellers are mostly measured in inches, for example 10” x 11” with the first number (10”) referring to the diameter and the second number (11”) referring to the pitch. So, what does that mean? The diameter refers to the distance the prop would make when spinning (distance traveled around the circle). This is measured by multiplying the distance between the end of the blade and the center of the prop or the hub. Pitch is a little more sticky (OK, pun intended this time); it is the theoretical progression that the propeller would make in one revolution. But, and this is where it gets sticky, water isn’t a solid so some slip occurs so the progression is actually slightly less.

Diameter is generally related to pitch. The lower the pitch the larger the diameter and the larger the diameter the lower the pitch. This makes sense because large heavy slow boats usually have lower pitched props with large diameters which provides the needed propulsion to move the boat. RIBs, which are faster and lighter generally need less thrust to get on a plane so as a result require smaller diameter props that have higher pitches.

Now that you have the basics covered, we’ll look at how to make sure you have the “right” prop on your boat in another posting. Stay tuned!

Posted by ribcraftusa at 03:52 PM | Permalink | Comments (0)

There was a comment to one of my postings from a month or so ago that raised the question of who was the first to dream up the concept of a RIB. I thought it was an excellent comment, so, where did the RIB come from? Its origins date back to around the turn of the 20th Century. To talk about the birth of RIBs, one has to discuss the advent of inflatable boats.

In large part, we have the Titanic to thank for our boats. With the loss of life of the Titanic in 1912 and the considerable losses experienced to US ships during World War I, the need for a solution was clear. After this, an international agreement was signed to provide safeguards for passenger safety aboard ships; creating the first SOLAS treaty. One of its major provisions was to ensure that every vessel had enough lifeboats to provide every person aboard the ship with a place. This was easy enough for cargo ships where there was minimal number of crew members and excess deck space, but for cruise ships and naval ships that had large passenger counts with almost no deck space this created a huge problem; where do you put the lifeboats? Enter inflatable boats…

After World War I, Goodyear (that’s right the U.S. tire company) found a way to join rubber to other materials. They made life rafts that had square shaped inflatable tubes with a rigid floor. Was this the birth of the first RIB? One could definitely argue yes. These rafts were to be stacked vertically on the ships and appeared to answer the storage problem – unfortunately, conservative thinking in the Navy prevented these boats from being developed fully.

Around the same time, across the pond, Pierre Debroutelle came up with a craft that utilized a U shaped inflatable tube. It was the first boat of its kind to be certified by a navy when the French Navy did so. During World War II, they added a wooden transom which was then patented.

During the second World War, everything changed; the need for inflatables increased and luckily so did the quality of the rubber used in the boats. Inflatables were used as lifeboats, to transport troops through shallow water, and to move torpedoes and other cargo. Additionally, their compactability made them easy to store and transport over land.

As with all great innovations, one company emerge; Zodiac. Their boats gained acceptance in the military and after World War II surplus inflatable boats were sold to the public, which continued their popularity in the recreational market. Zodiac quickly became the “generic” name when it came to inflatables.

In the 1950s, a French Naval officer by the name of Bombard combined the outboard engine, rigid floor and a boat shaped inflatable tube. Zodiac built the boat and a friend of Bombard, Jacques Cousteau began to use it. Cousteau was convinced by the performance of the boat and used it for years as a tender for his expeditions. As such, “Zodiac” became the word used for inflatable boats throughout Europe and quickly gained hold in the U.S.

The inflatable was so successful that Zodiac couldn’t keep up with the demand and they licensed production to several companies throughout Europe. By this time, people began “tweaking” the design to improve performance through rough water. They began playing with underwater inflatable hulls – the pre-cursor to the RIB.

The combination of a rigid hull and large inflatable tube was introduced in Great Britain in 1967 by Tony Lee-Elliott and patented by Admiral Hoare in 1969 after research and development at Atlanic College in Wales. RIBs were first introduced as lifeboats and rescue craft in England in 1970.

The rest as they say is history!

Posted by ribcraftusa at 11:58 AM | Permalink | Comments (0)