�� Buyer guide jetcraft.ru Boat Materials

	Aluminum	Fiberglass
Stiffness	Pound for pound as stiffas steel	Low. Needs reinforcement
Density	35% of steel	Low. Medium
Strength to weight Ratio	High	Low
Corrosion resistance in Salt Water	High	High
Osmosis Risk	Nil	High
Impact Resistance	Varies, depending upon grade (see below)	Brittle, very little. Once hit osmosis sets in
UV Resistance	High	Low - needs additives

As can be seen from the above, aluminum is superior as a construction material to

fiberglass for structural components as:

� It has high impact resistance

� There is no osmosis risk

� It has high rigidity

� It is lighter.

� It has a high strength to weight ratio.

Cost differences between glass and aluminum

Cost is a misleading comparison. Although pound for pound, aluminum is more

expensive than fiberglass, one needs substantially more fiberglass to obtain an equally

strong hull. For example, it takes about a 3,000- 4,000 lbs. of fiberglass to equal the

strength of a 1,000 pound aluminum hull. There are a few glass boat builders who build

this strong a hull. When you check their prices you will find they cost substantially more

than the aluminum hull.

Secondly, although the glass hull may initially be as strong, it will not have anywhere

near the impact resistance of aluminum.

Thirdly, although they may be able to handle an initial impact, the collision creates stress

cracks which allow osmosis to set in and the hull will deteriorate over time. Sun, stress

cracks and osmosis will all wreak havoc over time. Therefore, over time fiberglass�

strength deteriorates.

Fourth, all the added weight on the glass boat creates a severe performance, fuel

consumption and power plant penalty. To push the extra weight of a glass boat a larger,

heavier more expensive engine is required. In a bit of negative circle, bigger engines need

to be even bigger to push their own weight. Bigger engines burn more fuel which requires

even more weight for fuel storage and therefore an even bigger engine. More weight

makes for slower planning and more wallowing in big water. Ask any heavy duty glass

boat owner about the performance and operating cost of their boat.

Types of Aluminum

All welded aluminum boats are not created equal. Builders used different techniques and

different aluminum in their hulls. The following tells you what to look for:

	5052- H32 (��: 1 2)	5086- H32	6061-T6
Impact resistance	Soft, dents easily	Hard	Hardest
Tensile strength - Ultimate (where the metal breaks)	100%	129%	135%
Tensile strength - Yield (point to which metal bends and will not spring back)	100%	122%	152%
Ducility (ease of bending)	Easy Care must	be taken	Limited bending
Where used in Marathon	Sides, dashes	Bottom	Stringers, side trays

� Based on 1⁄4" thickness

� Figures are minimums. Actual samples will vary. 5052 tends to have the highest

variability.

From the chart above one can see why Marathon uses 6061 for the rub rail (gunwale) joint

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components and 5086 for bottoms. These are structural components that hold the boat

together. As boat powers through the water it (flexes). (Therefore) you want strength,

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hardness and little bending ability in your structural components. It is the structural

components that allow the boat to keep its integrity.

How does this affect you? Ask what is in the boat.. See how it is built. Ask what

materials go into it. Why ask? Some boats use 5052 in their bottoms. Others use minimal

thicknesses and designs in their stringers. If you are a light duty user, it probably does not

make much difference. However if you (put your boat through its paces) and push it a little,

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you will see the differences show up over time as one boat will develop cracks and the

other will not.

Material Thickness

This paper is talking about (heavy gauge) welded aluminum boats or what some call plate

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aluminum. The bottoms in these boats range (from 3/16� to 1⁄2�). Although some builders

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in this class use thinner bottoms in this class, most quality builders do not. Heavy gauge

welded boats usually see heavy use and requires the strength.

We are totally excluding boats of the light duty welded aluminum class, which are

basically a small step up from riveted boats. These builders use thin bottom material,

often 5052 and (reinforce) their boats completely differently than heavy gauge builders.

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Secondly, look at the material thickness and listen to the (explanations) as to what is used

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and why.

Boats: What to Look For. �� .

Introduction.

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Welded Boats in General:

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Our Bias

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Shop for a Manufacturer First

Look in the used boat ads

Judging the Production Staff

Manufacturer Reputation

Buying a Welded Boat for the First Time

Know the Boat you want

There is no Perfect Boat

Take a test ride.

Acceleration out of the hole

Bottom Width and Engine Size and Type.

Turning ability.

Stability at Speed: Porpoising versus bow down ��.

The Hook.

Spray Line?.

Boat Materials: Aluminum versus Fiberglass . ��

Cost differences between glass and aluminum . �� .

Types of Aluminum. �� .

Material Thickness

A Word about Welding

Suck marks, caterpillar tracks

Folded, �Origami� Construction

Hull Design

Delta Keel.

Deadrise (Vee)

Planing Hull Bottom Types ��

Constant deadrise V (including modified V hulls)

Flat bottom hulls (8 degrees deadrise or less)

Bow Entry Types. ��

Shallow Entry

Sharp Entry

Radius Bottom �� .

Lift Forces Explained.

Lateral Stability During Planing Turns

Bottom Shape and Jet Intake

Planing Ability

Reverse Chines

Bottom Width: Flair is not everything

No Trim.

Hull Construction Standards

Shallow Water Operation

No Exposed Drive

Safety

Thrust line and Center of Gravity In line

Propeller Torque is Different than Impeller Torque

Propulsion Resistance.

Power Plants

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Derived from Impeller Curves.

RPM Curve Derivation

Conclusion

Hull Resistance.

Boat attitude..

Jet Drives

How jets work .

Impeller

Cut.

Cut, Speed and Load

Wear ring.

Nozzle

Stator.

Hard Anodized.

Cavitation .

Intake

Seals

Mixed flow

Axial flow.

Outboard jet:

Inboard jets:

So, what is the best jet?

Corrosion.....

UHMW...

Jet Diagnosis...

Rattle at Idle (V8 boats)

Speed too low for a Given RPM.

Poor acceleration or �holeshot�, combined with high top end

Definitions.

Porpoising .

Chine walk: 9

Ram effect .

Static thrust:

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Spiridonov S. E.

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JETCRAFT. ��. �� . ��. ��. �� . �� . (Boats: What to Look For . Buyer guide)